KIP-164 — Add UnderMinIsrPartitionCount and per-partition UnderMinIsr metrics

Accepted Kafka 1.0 MetricsBroker

Adds a `UnderMinIsrPartitionCount` gauge metric at the broker level and per-partition `UnderMinIsr` indicators, tracking how many partition-leaders have fewer in-sync replicas than `min.insync.replicas`. The existing `UnderReplicatedPartitions` metric counts partitions below full replication but does not distinguish between partitions that merely lag versus partitions that will reject `acks=all` produce requests, making availability monitoring imprecise.

Details