KIP-427 — Add AtMinIsr topic partition category (new metric & TopicCommand option)

Accepted Kafka 2.3 MetricsBroker

KIP-427 adds an `AtMinIsr` partition category with a corresponding JMX metric and a `--at-min-isr` filter option for `kafka-topics.sh --describe`, identifying partitions whose ISR size equals `min.insync.replicas` exactly. The existing `UnderReplicatedPartitions` metric fires for any ISR shrinkage (including normal reassignments and broker restarts), making it too noisy to use as an actionable alert for the specific condition where one more ISR loss would cause `acks=all` produce failures.

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