NoteDBMigrator: allow to disable executor metrics

By default the WorkQueue executor used to run the online noteDb
migration is created with metrics-capability, so that the scheduling the
execution and the termination of submitted tasks could be monitored.

Upon termination of the ThreadPool executor, however, the metrics are
not removed from the metrics registry.

This is not just polluting the metrics registry with unnecessary
metrics, but it is also preventing from creating another queue with the
same name later within the same Gerrit runtime, causing the following
exception:

java.lang.IllegalArgumentException: A metric named
queue/<queue_name>/max_pool_size already exists

The root cause of this problem could be fixed by cherry-picking
I06d48535702a and explicitly de-register the metrics upon executor
termination, as attempted in Iaca9881cd46.

Unfortunately however, this cannot be done on stable-2.16, because it
would break binary-compatibility on all existing plugins that are using
metrics.

Allow to explicitly disable the NoteDBMigrator WorkQueue metrics
("RebuildChange"), so that subsequent online migrations can be triggered
without causing metrics collision.

Note that this is not a problem for the `migrate-to-notedb` program,
which is using `DisabledMetriMaker`, hence not emitting metrics at all.

Release-Notes: skip
Forward-Compatible: checked
Change-Id: Iaca9881cd467e32829d630aa0c259a8880e08c86

1 files changed, 13 insertions, 1 deletions

diff --git a/java/com/google/gerrit/server/notedb/rebuild/NoteDbMigrator.java b/java/com/google/gerrit/server/notedb/rebuild/NoteDbMigrator.java
index ec27ba27f8..ea8b9f5259 100644
--- a/java/com/google/gerrit/server/notedb/rebuild/NoteDbMigrator.java
+++ b/java/com/google/gerrit/server/notedb/rebuild/NoteDbMigrator.java
@@ -208,6 +208,7 @@ public class NoteDbMigrator implements AutoCloseable {
     private boolean autoMigrate;
     private boolean lockLooseRefs;
     private boolean verbose;
+    private boolean executorMetrics = true;
 
     @Inject
     Builder(
@@ -480,6 +481,17 @@ public class NoteDbMigrator implements AutoCloseable {
       return this;
     }
 
+    /**
+     * Set whether to enable the executor metrics.
+     *
+     * @param executorMetrics whether to enable the executor metrics (default true)
+     * @return this
+     */
+    public Builder setEnableExecutorMetrics(boolean executorMetrics) {
+      this.executorMetrics = executorMetrics;
+      return this;
+    }
+
     public NoteDbMigrator build() throws MigrationException {
       return new NoteDbMigrator(
           sitePaths,
@@ -498,7 +510,7 @@ public class NoteDbMigrator implements AutoCloseable {
           listeners,
           threads > 1
               ? MoreExecutors.listeningDecorator(
-                  workQueue.createQueue(threads, "RebuildChange", true))
+                  workQueue.createQueue(threads, "RebuildChange", executorMetrics))
               : MoreExecutors.newDirectExecutorService(),
           projects,
           skipProjects,