KAFKA-5544; The LastStableOffsetLag metric should be removed when partition is deleted

Author: Dong Lin <lindong28@gmail.com>

Reviewers: Ismael Juma <ismael@juma.me.uk>

Closes #3463 from lindong28/KAFKA-5544

core/src/main/scala/kafka/cluster/Partition.scala

@@ -561,6 +561,7 @@ class Partition(val topic: String,
     removeMetric("UnderReplicated", tags)
     removeMetric("InSyncReplicasCount", tags)
     removeMetric("ReplicasCount", tags)
+    removeMetric("LastStableOffsetLag", tags)
   }
 
   override def equals(that: Any): Boolean = that match {