KIP-973 — Expose per topic replication rate metrics

Discussion MetricsBroker

Corrects ReplicationBytesInPerSec and ReplicationBytesOutPerSec MBeans to report per-topic metrics (adding a topic tag) rather than only aggregate broker-level metrics, aligning implementation with documentation. Tools like Cruise Control assumed these metrics were per-topic but the implementation only tracked broker-level aggregates, producing incorrect workload models.

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