KIP-1093 — Add more observability for MirrorMaker2

Discussion MirrorMakerMetrics

Adds high-level MirrorMaker2 metrics for `topic-count`, `partition-count` per topic, `consumer-group-count`, and `consumer-group-offset` to complement the existing record-level metrics. The current MM2 metrics cover data throughput and replication latency but provide no visibility into the aggregate number of topics and consumer groups being synchronized, making capacity planning and correctness verification opaque.

Details