KIP-1184 — Add connector name and task index tags to MirrorMaker tasks' metrics

Discussion MirrorMakerMetrics

Adds `connector.name` and `task.index` tags to the `Metrics` instance created per MirrorMaker task, and uses `source` + `target` cluster as default tags, to prevent metric name collisions across multiple tasks and cluster directions. MirrorMaker tasks currently create `Metrics` instances without tags, causing `kafka_metrics_count` (and other tag-less metrics) to collide when multiple tasks or multiple replication directions share the same JVM.

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