Allow different data reservoirs for metrics

The default reservoir for collecting data with DropWizard may not
be suitable for all types of metrics collection.

The default ExponentiallyDecayingReservoir is typically suitable
for data with a lots of values per unit whilst shows inaccurancy
for smaller data sets spread over a longer period of time.

Introduce a new DropWizardReservoirProvider which reads the
reservoir type from gerrit.config in a new section called [metrics].
The reservoir can be subsequently configured in the subsection
[metrics "<reservoir>"] and its parameters tailored to the Gerrit
admin's metrics collection system.

For example, when using Prometheus scraper on a 1 mins interval
basis, it is possible to swap the default reservoir with a more
suitable SlidingTimeWindow:

[metrics]
  reservoir = SlidingTimeWindow

[metrics "SlidingTimeWindow"]
  window = 1 min

Release-Notes: introduce metrics configuration for different data reservoirs
Change-Id: Iaf73bc5c1a73423d06f423ea813df2d96e151f9e

11 files changed, 393 insertions, 6 deletions

diff --git a/Documentation/config-gerrit.txt b/Documentation/config-gerrit.txt
index dbe390957e..8ad5cef739 100644
--- a/Documentation/config-gerrit.txt
+++ b/Documentation/config-gerrit.txt
@@ -4008,6 +4008,46 @@ If set to true, log files are rotated daily at midnight (GMT).
 +
 Defaults to true.
 
+[[metrics]]
+=== Section metrics
+
+[[metrics.reservoir]]metrics.reservoir::
++
+The type of data reservoir used by the metrics system to calculate the percentile
+values for timers and histograms.
+It can be set to one of the following values:
++
+* ExponentiallyDecaying: An exponentially-decaying random reservoir based on
+  Cormode et al's forward-decaying priority reservoir sampling method to produce
+  a statistically representative sampling reservoir, exponentially biased towards
+  newer entries.
+* SlidingTimeWindowArray: A sliding window that stores only the measurements made
+  in the last window using chunks of 512 samples.
+* SlidingTimeWindow: A sliding window that stores only the measurements made in
+  the last window using a skip list.
+* SlidingWindow: A sliding window that stores only the last measurements.
+* Uniform: A random sampling reservoir that uses Vitter's Algorithm R to produce
+  a statistically representative sample.
++
+Defaults to ExponentiallyDecaying.
+
+[[metrics.ExponentiallyDecaying.alpha]]metrics.ExponentiallyDecaying.alpha::
++
+The exponential decay factor; the higher this is, the more biased the reservoir
+will be towards newer values.
+
+[[metrics.reservoirType.size]]metrics.<reservoirType>.size::
++
+The number of samples to keep in the reservoir. Applies to all reservoir types
+except the sliding time-based ones.
++
+Defaults to 1028.
+
+[[metrics.reservoirType.window]]metrics.<reservoirType>.window::
++
+The window of time for keeping data in the reservoir. It only applies to sliding
+time-based reservoir types.
+
 [[mimetype]]
 === Section mimetype
 
diff --git a/java/com/google/gerrit/acceptance/InMemoryTestingDatabaseModule.java b/java/com/google/gerrit/acceptance/InMemoryTestingDatabaseModule.java
index de9a43d6d6..c2b21fbb11 100644
--- a/java/com/google/gerrit/acceptance/InMemoryTestingDatabaseModule.java
+++ b/java/com/google/gerrit/acceptance/InMemoryTestingDatabaseModule.java
@@ -21,7 +21,9 @@ import com.google.gerrit.exceptions.StorageException;
 import com.google.gerrit.extensions.events.LifecycleListener;
 import com.google.gerrit.lifecycle.LifecycleModule;
 import com.google.gerrit.metrics.MetricMaker;
+import com.google.gerrit.metrics.MetricsReservoirConfig;
 import com.google.gerrit.server.config.GerritServerConfig;
+import com.google.gerrit.server.config.MetricsReservoirConfigImpl;
 import com.google.gerrit.server.config.SitePath;
 import com.google.gerrit.server.config.SitePaths;
 import com.google.gerrit.server.config.TrackingFooters;
@@ -32,6 +34,7 @@ import com.google.gerrit.server.schema.SchemaModule;
 import com.google.gerrit.testing.InMemoryRepositoryManager;
 import com.google.inject.Inject;
 import com.google.inject.ProvisionException;
+import com.google.inject.Scopes;
 import java.io.IOException;
 import java.nio.file.Files;
 import java.nio.file.Path;
@@ -63,6 +66,7 @@ class InMemoryTestingDatabaseModule extends LifecycleModule {
       bind(InMemoryRepositoryManager.class).in(SINGLETON);
     }
 
+    bind(MetricsReservoirConfig.class).to(MetricsReservoirConfigImpl.class).in(Scopes.SINGLETON);
     bind(MetricMaker.class).to(TestMetricMaker.class);
 
     listener().to(CreateSchema.class);
diff --git a/java/com/google/gerrit/metrics/MetricsReservoirConfig.java b/java/com/google/gerrit/metrics/MetricsReservoirConfig.java
new file mode 100644
index 0000000000..455dfc0829
--- /dev/null
+++ b/java/com/google/gerrit/metrics/MetricsReservoirConfig.java
@@ -0,0 +1,33 @@
+// Copyright (C) 2022 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics;
+
+import java.time.Duration;
+
+/** Configuration of the Metrics' reservoir type and size. */
+public interface MetricsReservoirConfig {
+
+  /** @return the reservoir type. */
+  ReservoirType reservoirType();
+
+  /** @return the reservoir window duration. */
+  Duration reservoirWindow();
+
+  /** @return the number of samples that the reservoir can contain */
+  int reservoirSize();
+
+  /** @return the alpha parameter of the ExponentiallyDecaying reservoir */
+  double reservoirAlpha();
+}
diff --git a/java/com/google/gerrit/metrics/ReservoirType.java b/java/com/google/gerrit/metrics/ReservoirType.java
new file mode 100644
index 0000000000..fe8975205f
--- /dev/null
+++ b/java/com/google/gerrit/metrics/ReservoirType.java
@@ -0,0 +1,24 @@
+// Copyright (C) 2022 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics;
+
+/** Type of reservoir for collecting metrics into. */
+public enum ReservoirType {
+  ExponentiallyDecaying,
+  SlidingTimeWindowArray,
+  SlidingTimeWindow,
+  SlidingWindow,
+  Uniform;
+}
diff --git a/java/com/google/gerrit/metrics/dropwizard/DropWizardMetricMaker.java b/java/com/google/gerrit/metrics/dropwizard/DropWizardMetricMaker.java
index fcba0eeb3c..32be18d981 100644
--- a/java/com/google/gerrit/metrics/dropwizard/DropWizardMetricMaker.java
+++ b/java/com/google/gerrit/metrics/dropwizard/DropWizardMetricMaker.java
@@ -18,8 +18,10 @@ import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.gerrit.metrics.dropwizard.MetricResource.METRIC_KIND;
 import static com.google.gerrit.server.config.ConfigResource.CONFIG_KIND;
 
+import com.codahale.metrics.Histogram;
 import com.codahale.metrics.Metric;
 import com.codahale.metrics.MetricRegistry;
+import com.codahale.metrics.Timer;
 import com.google.common.collect.FluentIterable;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
@@ -41,6 +43,7 @@ import com.google.gerrit.metrics.Histogram1;
 import com.google.gerrit.metrics.Histogram2;
 import com.google.gerrit.metrics.Histogram3;
 import com.google.gerrit.metrics.MetricMaker;
+import com.google.gerrit.metrics.MetricsReservoirConfig;
 import com.google.gerrit.metrics.Timer0;
 import com.google.gerrit.metrics.Timer1;
 import com.google.gerrit.metrics.Timer2;
@@ -48,10 +51,12 @@ import com.google.gerrit.metrics.Timer3;
 import com.google.gerrit.metrics.proc.JGitMetricModule;
 import com.google.gerrit.metrics.proc.ProcMetricModule;
 import com.google.gerrit.server.cache.CacheMetrics;
+import com.google.gerrit.server.config.MetricsReservoirConfigImpl;
 import com.google.inject.Inject;
 import com.google.inject.Scopes;
 import com.google.inject.Singleton;
 import java.util.Map;
+import java.util.Optional;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.TimeUnit;
@@ -65,8 +70,25 @@ import java.util.regex.Pattern;
 @Singleton
 public class DropWizardMetricMaker extends MetricMaker {
   public static class ApiModule extends RestApiModule {
+    private final Optional<MetricsReservoirConfig> metricsReservoirConfig;
+
+    public ApiModule(MetricsReservoirConfig metricsReservoirConfig) {
+      this.metricsReservoirConfig = Optional.of(metricsReservoirConfig);
+    }
+
+    public ApiModule() {
+      this.metricsReservoirConfig = Optional.empty();
+    }
+
     @Override
     protected void configure() {
+      if (metricsReservoirConfig.isPresent()) {
+        bind(MetricsReservoirConfig.class).toInstance(metricsReservoirConfig.get());
+      } else {
+        bind(MetricsReservoirConfig.class)
+            .to(MetricsReservoirConfigImpl.class)
+            .in(Scopes.SINGLETON);
+      }
       bind(MetricRegistry.class).in(Scopes.SINGLETON);
       bind(DropWizardMetricMaker.class).in(Scopes.SINGLETON);
       bind(MetricMaker.class).to(DropWizardMetricMaker.class);
@@ -89,12 +111,14 @@ public class DropWizardMetricMaker extends MetricMaker {
   private final MetricRegistry registry;
   private final Map<String, BucketedMetric> bucketed;
   private final Map<String, ImmutableMap<String, String>> descriptions;
+  private final MetricsReservoirConfig reservoirConfig;
 
   @Inject
-  DropWizardMetricMaker(MetricRegistry registry) {
+  DropWizardMetricMaker(MetricRegistry registry, MetricsReservoirConfig reservoirConfig) {
     this.registry = registry;
     this.bucketed = new ConcurrentHashMap<>();
     this.descriptions = new ConcurrentHashMap<>();
+    this.reservoirConfig = reservoirConfig;
   }
 
   Iterable<String> getMetricNames() {
@@ -222,7 +246,9 @@ public class DropWizardMetricMaker extends MetricMaker {
   }
 
   TimerImpl newTimerImpl(String name) {
-    return new TimerImpl(name, registry.timer(name));
+    return new TimerImpl(
+        name,
+        registry.timer(name, () -> new Timer(DropWizardReservoirProvider.get(reservoirConfig))));
   }
 
   @Override
@@ -271,7 +297,10 @@ public class DropWizardMetricMaker extends MetricMaker {
   }
 
   HistogramImpl newHistogramImpl(String name) {
-    return new HistogramImpl(name, registry.histogram(name));
+    return new HistogramImpl(
+        name,
+        registry.histogram(
+            name, () -> new Histogram(DropWizardReservoirProvider.get(reservoirConfig))));
   }
 
   @Override
diff --git a/java/com/google/gerrit/metrics/dropwizard/DropWizardReservoirProvider.java b/java/com/google/gerrit/metrics/dropwizard/DropWizardReservoirProvider.java
new file mode 100644
index 0000000000..30890689cb
--- /dev/null
+++ b/java/com/google/gerrit/metrics/dropwizard/DropWizardReservoirProvider.java
@@ -0,0 +1,52 @@
+// Copyright (C) 2022 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics.dropwizard;
+
+import com.codahale.metrics.ExponentiallyDecayingReservoir;
+import com.codahale.metrics.Reservoir;
+import com.codahale.metrics.SlidingTimeWindowArrayReservoir;
+import com.codahale.metrics.SlidingTimeWindowReservoir;
+import com.codahale.metrics.SlidingWindowReservoir;
+import com.codahale.metrics.UniformReservoir;
+import com.google.gerrit.metrics.MetricsReservoirConfig;
+import com.google.gerrit.metrics.ReservoirType;
+import java.util.concurrent.TimeUnit;
+
+class DropWizardReservoirProvider {
+
+  private DropWizardReservoirProvider() {}
+
+  static Reservoir get(MetricsReservoirConfig config) {
+    ReservoirType reservoirType = config.reservoirType();
+    switch (reservoirType) {
+      case ExponentiallyDecaying:
+        return new ExponentiallyDecayingReservoir(config.reservoirSize(), config.reservoirAlpha());
+      case SlidingTimeWindowArray:
+        return new SlidingTimeWindowArrayReservoir(
+            config.reservoirWindow().toMillis(), TimeUnit.MILLISECONDS);
+      case SlidingTimeWindow:
+        return new SlidingTimeWindowReservoir(
+            config.reservoirWindow().toMillis(), TimeUnit.MILLISECONDS);
+      case SlidingWindow:
+        return new SlidingWindowReservoir(config.reservoirSize());
+      case Uniform:
+        return new UniformReservoir(config.reservoirSize());
+
+      default:
+        throw new IllegalArgumentException(
+            "Unsupported metrics reservoir type " + reservoirType.name());
+    }
+  }
+}
diff --git a/java/com/google/gerrit/server/config/MetricsReservoirConfigImpl.java b/java/com/google/gerrit/server/config/MetricsReservoirConfigImpl.java
new file mode 100644
index 0000000000..ac3c53acf7
--- /dev/null
+++ b/java/com/google/gerrit/server/config/MetricsReservoirConfigImpl.java
@@ -0,0 +1,96 @@
+// Copyright (C) 2022 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.config;
+
+import com.google.gerrit.metrics.MetricsReservoirConfig;
+import com.google.gerrit.metrics.ReservoirType;
+import com.google.inject.Inject;
+import com.google.inject.Singleton;
+import java.time.Duration;
+import java.util.Optional;
+import java.util.concurrent.TimeUnit;
+import org.eclipse.jgit.lib.Config;
+
+/** Define metrics reservoir settings based on gerrit.config */
+@Singleton
+public class MetricsReservoirConfigImpl implements MetricsReservoirConfig {
+  private static final double RESERVOIR_ALPHA_DEFAULT = 0.015;
+  private static final int METRICS_RESERVOIR_SIZE_DEFAULT = 1028;
+  private static final long METRICS_RESERVOIR_WINDOW_MSEC_DEFAULT = 60000L;
+  private static final String METRICS_SECTION = "metrics";
+  private static final String METRICS_RESERVOIR = "reservoir";
+
+  private final ReservoirType reservoirType;
+
+  private final Duration reservoirWindow;
+  private final int reservoirSize;
+  private final double reservoirAlpha;
+
+  @Inject
+  MetricsReservoirConfigImpl(@GerritServerConfig Config gerritConfig) {
+    this.reservoirType =
+        gerritConfig.getEnum(
+            METRICS_SECTION, null, METRICS_RESERVOIR, ReservoirType.ExponentiallyDecaying);
+
+    reservoirWindow =
+        Duration.ofMillis(
+            ConfigUtil.getTimeUnit(
+                gerritConfig,
+                METRICS_SECTION,
+                reservoirType.name(),
+                "window",
+                METRICS_RESERVOIR_WINDOW_MSEC_DEFAULT,
+                TimeUnit.MILLISECONDS));
+    reservoirSize =
+        gerritConfig.getInt(
+            METRICS_SECTION, reservoirType.name(), "size", METRICS_RESERVOIR_SIZE_DEFAULT);
+    reservoirAlpha =
+        Optional.ofNullable(gerritConfig.getString(METRICS_SECTION, reservoirType.name(), "alpha"))
+            .map(Double::parseDouble)
+            .orElse(RESERVOIR_ALPHA_DEFAULT);
+  }
+
+  /* (non-Javadoc)
+   * @see com.google.gerrit.server.config.MetricsConfig#reservoirType()
+   */
+  @Override
+  public ReservoirType reservoirType() {
+    return reservoirType;
+  }
+
+  /* (non-Javadoc)
+   * @see com.google.gerrit.server.config.MetricsConfig#reservoirWindow()
+   */
+  @Override
+  public Duration reservoirWindow() {
+    return reservoirWindow;
+  }
+
+  /* (non-Javadoc)
+   * @see com.google.gerrit.server.config.MetricsConfig#reservoirSize()
+   */
+  @Override
+  public int reservoirSize() {
+    return reservoirSize;
+  }
+
+  /* (non-Javadoc)
+   * @see com.google.gerrit.server.config.MetricsConfig#reservoirAlpha()
+   */
+  @Override
+  public double reservoirAlpha() {
+    return reservoirAlpha;
+  }
+}
diff --git a/javatests/com/google/gerrit/metrics/dropwizard/BUILD b/javatests/com/google/gerrit/metrics/dropwizard/BUILD
index 98d12b2e7b..e236f30ce5 100644
--- a/javatests/com/google/gerrit/metrics/dropwizard/BUILD
+++ b/javatests/com/google/gerrit/metrics/dropwizard/BUILD
@@ -6,7 +6,10 @@ junit_tests(
     tags = ["metrics"],
     visibility = ["//visibility:public"],
     deps = [
+        "//java/com/google/gerrit/metrics",
         "//java/com/google/gerrit/metrics/dropwizard",
+        "//lib/mockito",
         "//lib/truth",
+        "@dropwizard-core//jar",
     ],
 )
diff --git a/javatests/com/google/gerrit/metrics/dropwizard/DropWizardMetricMakerTest.java b/javatests/com/google/gerrit/metrics/dropwizard/DropWizardMetricMakerTest.java
index 9b21bf6c01..5777779513 100644
--- a/javatests/com/google/gerrit/metrics/dropwizard/DropWizardMetricMakerTest.java
+++ b/javatests/com/google/gerrit/metrics/dropwizard/DropWizardMetricMakerTest.java
@@ -15,12 +15,33 @@
 package com.google.gerrit.metrics.dropwizard;
 
 import static com.google.common.truth.Truth.assertThat;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
 
+import com.codahale.metrics.MetricRegistry;
+import com.google.gerrit.metrics.Description;
+import com.google.gerrit.metrics.MetricsReservoirConfig;
+import com.google.gerrit.metrics.ReservoirType;
+import org.junit.Before;
 import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.junit.MockitoJUnitRunner;
 
+@RunWith(MockitoJUnitRunner.class)
 public class DropWizardMetricMakerTest {
-  DropWizardMetricMaker metrics =
-      new DropWizardMetricMaker(null /* MetricRegistry unused in tests */);
+
+  @Mock MetricsReservoirConfig reservoirConfigMock;
+
+  MetricRegistry registry;
+
+  DropWizardMetricMaker metrics;
+
+  @Before
+  public void setupMocks() {
+    registry = new MetricRegistry();
+    metrics = new DropWizardMetricMaker(registry, reservoirConfigMock);
+  }
 
   @Test
   public void shouldSanitizeUnwantedChars() throws Exception {
@@ -41,4 +62,15 @@ public class DropWizardMetricMakerTest {
     assertThat(metrics.sanitizeMetricName("metric//")).isEqualTo("metric");
     assertThat(metrics.sanitizeMetricName("metric/submetric/")).isEqualTo("metric/submetric");
   }
+
+  @Test
+  public void shouldRequestForReservoirForNewTimer() throws Exception {
+    when(reservoirConfigMock.reservoirType()).thenReturn(ReservoirType.ExponentiallyDecaying);
+
+    metrics.newTimer(
+        "foo",
+        new Description("foo description").setCumulative().setUnit(Description.Units.MILLISECONDS));
+
+    verify(reservoirConfigMock).reservoirType();
+  }
 }
diff --git a/javatests/com/google/gerrit/metrics/dropwizard/DropWizardReservoirProviderTest.java b/javatests/com/google/gerrit/metrics/dropwizard/DropWizardReservoirProviderTest.java
new file mode 100644
index 0000000000..6402b53ad4
--- /dev/null
+++ b/javatests/com/google/gerrit/metrics/dropwizard/DropWizardReservoirProviderTest.java
@@ -0,0 +1,64 @@
+// Copyright (C) 2022 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.metrics.dropwizard;
+
+import static com.google.common.truth.Truth.assertThat;
+import static org.mockito.Mockito.when;
+
+import com.codahale.metrics.ExponentiallyDecayingReservoir;
+import com.codahale.metrics.SlidingTimeWindowArrayReservoir;
+import com.codahale.metrics.SlidingTimeWindowReservoir;
+import com.codahale.metrics.SlidingWindowReservoir;
+import com.codahale.metrics.UniformReservoir;
+import com.google.gerrit.metrics.MetricsReservoirConfig;
+import com.google.gerrit.metrics.ReservoirType;
+import java.time.Duration;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.junit.MockitoJUnitRunner;
+
+@RunWith(MockitoJUnitRunner.class)
+public class DropWizardReservoirProviderTest {
+  private static final int SLIDING_WINDOW_INTERVAL = 1;
+  private static final int SLIDING_WINDOW_SIZE = 256;
+
+  @Mock private MetricsReservoirConfig configMock;
+
+  @Test
+  public void shouldInstantiateReservoirProviderBasedOnMetricsConfig() {
+    when(configMock.reservoirType()).thenReturn(ReservoirType.ExponentiallyDecaying);
+    assertThat(DropWizardReservoirProvider.get(configMock))
+        .isInstanceOf(ExponentiallyDecayingReservoir.class);
+
+    when(configMock.reservoirType()).thenReturn(ReservoirType.SlidingTimeWindow);
+    when(configMock.reservoirWindow()).thenReturn(Duration.ofMinutes(1));
+    assertThat(DropWizardReservoirProvider.get(configMock))
+        .isInstanceOf(SlidingTimeWindowReservoir.class);
+
+    when(configMock.reservoirType()).thenReturn(ReservoirType.SlidingTimeWindowArray);
+    when(configMock.reservoirWindow()).thenReturn(Duration.ofMinutes(SLIDING_WINDOW_INTERVAL));
+    assertThat(DropWizardReservoirProvider.get(configMock))
+        .isInstanceOf(SlidingTimeWindowArrayReservoir.class);
+
+    when(configMock.reservoirType()).thenReturn(ReservoirType.SlidingWindow);
+    when(configMock.reservoirSize()).thenReturn(SLIDING_WINDOW_SIZE);
+    assertThat(DropWizardReservoirProvider.get(configMock))
+        .isInstanceOf(SlidingWindowReservoir.class);
+
+    when(configMock.reservoirType()).thenReturn(ReservoirType.Uniform);
+    assertThat(DropWizardReservoirProvider.get(configMock)).isInstanceOf(UniformReservoir.class);
+  }
+}
diff --git a/javatests/com/google/gerrit/metrics/proc/ProcMetricModuleTest.java b/javatests/com/google/gerrit/metrics/proc/ProcMetricModuleTest.java
index 33919e7ad0..ea89ae9d92 100644
--- a/javatests/com/google/gerrit/metrics/proc/ProcMetricModuleTest.java
+++ b/javatests/com/google/gerrit/metrics/proc/ProcMetricModuleTest.java
@@ -17,6 +17,7 @@ package com.google.gerrit.metrics.proc;
 import static com.google.common.truth.Truth.assertThat;
 import static com.google.common.truth.Truth.assertWithMessage;
 import static com.google.gerrit.testing.GerritJUnit.assertThrows;
+import static org.mockito.Mockito.mock;
 
 import com.codahale.metrics.Counter;
 import com.codahale.metrics.Gauge;
@@ -31,7 +32,9 @@ import com.google.gerrit.metrics.Description;
 import com.google.gerrit.metrics.Description.FieldOrdering;
 import com.google.gerrit.metrics.Field;
 import com.google.gerrit.metrics.MetricMaker;
+import com.google.gerrit.metrics.MetricsReservoirConfig;
 import com.google.gerrit.metrics.dropwizard.DropWizardMetricMaker;
+import com.google.inject.AbstractModule;
 import com.google.inject.Guice;
 import com.google.inject.Inject;
 import com.google.inject.Injector;
@@ -179,7 +182,14 @@ public class ProcMetricModuleTest {
 
   @Before
   public void setup() {
-    Injector injector = Guice.createInjector(new DropWizardMetricMaker.ApiModule());
+    Injector injector =
+        Guice.createInjector(
+            new AbstractModule() {
+              @Override
+              protected void configure() {
+                install(new DropWizardMetricMaker.ApiModule(mock(MetricsReservoirConfig.class)));
+              }
+            });
 
     LifecycleManager mgr = new LifecycleManager();
     mgr.add(injector);