KIP-352 — Distinguish URPs caused by reassignment

Accepted Kafka 2.5 BrokerAdmin

Adds a new under-replicated metric that excludes replicas catching up during an active reassignment, and a separate metric specifically for reassignment-lagging replicas. During reassignment, the standard UnderReplicatedPartitions metric fires even when desired replication factor is met, making it impossible to use URP alerts without false positives during planned reassignments.

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