KIP-1355 — In-rack traffic observability for clients and brokers

Discussion MetricsProducerConsumer

Adds in-rack traffic counters mirroring existing byte metrics: byte-in-rack-rate/total on the producer, bytes-consumed-in-rack-rate/total on the consumer per client and per topic, and ReplicationBytesInInRackPerSec / ReplicationBytesOutInRackPerSec on the broker. Follower fetching, rack-aware partitioning, and Diskless Topics all aim to keep traffic inside a rack, but nothing reports how much traffic actually stays local, leaving operators unable to validate the configuration or attribute cross-AZ transfer cost.

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