KIP-861 — Add BytesIn/BytesOut per listener metrics

Discussion MetricsBroker

Proposes BytesIn and BytesOut metrics tagged per listener name, enabling network billing and monitoring at the listener boundary (e.g., separating external client traffic from inter-broker replication traffic). Existing BrokerTopicMetrics aggregate bytes across all listeners and exclude protocol overhead (heartbeats, authentication), making accurate per-listener accounting impossible.

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