KIP-96 — Add per partition metrics for in-sync and assigned replica count

Accepted Kafka 1.0 MetricsBroker

Adds two per-partition JMX gauge metrics exposed by the partition leader: `InSyncReplicasCount` and `ReplicasCount` (total assigned replicas), tagged by topic and partition. The existing metrics only provide cluster-wide or broker-wide replica health summaries; operators cannot identify which specific partition has lost ISR members without querying ZooKeeper or using the AdminClient.

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