KIP-971 — Expose replication-record-lag MirrorMaker2 metric

Discussion MirrorMakerMetrics

Adds replication-record-lag metrics (max/min/avg) to MirrorMaker2's MirrorSourceTask, computed as the difference between the source partition's LEO and the last replicated offset (LRO). No built-in metric existed to directly quantify how many records remained to be replicated, forcing operators to infer lag from byte-lag proxies.

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