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Add the following two yammer metrics (and resulting JMX metrics)

- kafka.server:name=UnavailablePartitionsUnavailablePartitionCount,type=ReplicaManager

There is one such gauge per-broker

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- Add the yammer gauge Unavailable to the kafka.cluster.Partition class, similar to the existing "UnderReplicated" metric metric.

The value of this metric is 1 if the broker is leader of this partition AND the number of in-sync replicas of this partition < min ISR of this partition. Otherwise it is 0.

- Add the yammer gauge UnavailablePartitions UnavailablePartitionCount to the kafka.server.ReplicaManager class, similar to the existing "UnderReplicatedPartitions" metric metric.

The value of this metric is the total number of leader partitions on this broker which are "Unavailable"  (as defined above).

Compatibility, Deprecation, and Migration Plan

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