KAFKA-9599 create unique sensor to record group rebalance (#8159)

The "offset deletion" and "group rebalance" should not be recorded by the same sensor since they are totally different.

The code is introduced by #7276.

Reviewers: David Jacot <djacot@confluent.io>, Guozhang Wang <wangguoz@gmail.com>

core/src/main/scala/kafka/coordinator/group/GroupCoordinator.scala

@@ -74,7 +74,7 @@ class GroupCoordinator(val brokerId: Int,
       "group-coordinator-metrics",
       "The total number of administrative deleted offsets")))
 
-  val groupCompletedRebalanceSensor = metrics.sensor("OffsetDeletions")
+  val groupCompletedRebalanceSensor = metrics.sensor("CompletedRebalances")
 
   groupCompletedRebalanceSensor.add(new Meter(
     metrics.metricName("group-completed-rebalance-rate",