Add feature flag for undesired allocation tracker (#137717)

Author: nicktindall

server/src/main/java/org/elasticsearch/cluster/routing/allocation/allocator/UndesiredAllocationsTracker.java

@@ -16,6 +16,7 @@
 import org.elasticsearch.common.settings.ClusterSettings;
 import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.time.TimeProvider;
+import org.elasticsearch.common.util.FeatureFlag;
 import org.elasticsearch.core.TimeValue;
 import org.elasticsearch.index.shard.ShardId;
 import org.elasticsearch.logging.LogManager;
@@ -40,6 +41,7 @@ public class UndesiredAllocationsTracker {
     private static final Logger logger = LogManager.getLogger(UndesiredAllocationsTracker.class);
 
     private static final TimeValue FIVE_MINUTES = timeValueMinutes(5);
+    private static final FeatureFlag UNDESIRED_ALLOCATION_TRACKER_ENABLED = new FeatureFlag("undesired_allocation_tracker");
 
     /**
      * Warning logs will be periodically written if we see a shard that's been in an undesired allocation for this long
@@ -68,7 +70,7 @@ public class UndesiredAllocationsTracker {
      */
     public static final Setting<Integer> MAX_UNDESIRED_ALLOCATIONS_TO_TRACK = Setting.intSetting(
         "cluster.routing.allocation.desired_balance.undesired_duration_logging.max_to_track",
-        0,
+        UNDESIRED_ALLOCATION_TRACKER_ENABLED.isEnabled() ? 10 : 0,
         0,
         100,
         Setting.Property.Dynamic,