[LI-HOTFIX] Add OneAboveMinIsrCounter sensor. (#71)

TICKET =
LI_DESCRIPTION = Add the OneAboveMinIsrCounter metrics to help determining the good timing of broker rolling bounce.
EXIT_CRITERIA = MANUAL [""]

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

@@ -180,6 +180,15 @@ class Partition(val topicPartition: TopicPartition,
     }
   }
 
+  def isOneAboveMinIsr: Boolean = {
+    leaderReplicaIfLocal match {
+      case Some(leaderReplica) =>
+        inSyncReplicas.size == leaderReplica.log.get.config.minInSyncReplicas + 1
+      case None =>
+        false
+    }
+  }
+
   /**
     * Create the future replica if 1) the current replica is not in the given log directory and 2) the future replica
     * does not exist. This method assumes that the current replica has already been created.

core/src/main/scala/kafka/server/ReplicaManager.scala

@@ -252,6 +252,12 @@ class ReplicaManager(val config: KafkaConfig,
       def value = leaderPartitionsIterator.count(_.isAtMinIsr)
     }
   )
+  val oneAboveMinIsrPartitionCount = newGauge(
+    "OneAboveMinIsrPartitionCount",
+    new Gauge[Int] {
+      def value = leaderPartitionsIterator.count(_.isOneAboveMinIsr)
+    }
+  )
 
   val recompressionCount = newGauge(
     "recompressionCount",