[segment replication] Avoid skewed segment replication lag metric

CHANGELOG.md

@@ -35,6 +35,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Unwrap singleton DocValues in global ordinal value source of composite histogram aggregation ([#17740](https://github.com/opensearch-project/OpenSearch/pull/17740))
 - Unwrap singleton DocValues in date histogram aggregation. ([#17643](https://github.com/opensearch-project/OpenSearch/pull/17643))
 - Introduce 512 byte limit to search and ingest pipeline IDs ([#17786](https://github.com/opensearch-project/OpenSearch/pull/17786))
+- Avoid skewed segment replication lag metric ([#17831](https://github.com/opensearch-project/OpenSearch/pull/17831))
 
 ### Dependencies
 - Bump `com.nimbusds:nimbus-jose-jwt` from 9.41.1 to 10.0.2 ([#17607](https://github.com/opensearch-project/OpenSearch/pull/17607), [#17669](https://github.com/opensearch-project/OpenSearch/pull/17669))

server/src/main/java/org/opensearch/index/seqno/ReplicationTracker.java

@@ -1307,7 +1307,8 @@ public synchronized void startReplicationLagTimers(ReplicationCheckpoint checkpo
                     && replicationGroup.getUnavailableInSyncShards().contains(allocationId) == false
                     && shouldSkipReplicationTimer(e.getKey()) == false
                     && latestReplicationCheckpoint.isAheadOf(cps.visibleReplicationCheckpoint)
-                    && cps.checkpointTimers.containsKey(latestReplicationCheckpoint)) {
+                    && cps.checkpointTimers.containsKey(latestReplicationCheckpoint)
+                    && cps.checkpointTimers.get(latestReplicationCheckpoint).startTime() == 0) {
                     cps.checkpointTimers.get(latestReplicationCheckpoint).start();
                 }
             });

server/src/test/java/org/opensearch/index/seqno/ReplicationTrackerTests.java

@@ -1870,6 +1870,8 @@ public void testSegmentReplicationCheckpointTracking() {
 
         tracker.setLatestReplicationCheckpoint(initialCheckpoint);
         tracker.startReplicationLagTimers(initialCheckpoint);
+        // retry start replication lag timers
+        tracker.startReplicationLagTimers(initialCheckpoint);
         tracker.setLatestReplicationCheckpoint(secondCheckpoint);
         tracker.startReplicationLagTimers(secondCheckpoint);
         tracker.setLatestReplicationCheckpoint(thirdCheckpoint);