KIP-1280 — Update MirrorMaker to use KIP-877 to emit metrics

Accepted Kafka 4.3 MirrorMakerMetrics

Updates MirrorSourceConnector and MirrorCheckpointConnector to register their metrics via the KIP-877 connector metrics API instead of creating private Metrics instances with their own MetricsReporter instances. The previous workaround—each connector instantiating its own Metrics object—bypassed standard metrics plumbing and was never available for MirrorHeartbeatConnector at all.

Details