KIP-484 — Expose metrics for group and transaction metadata loading duration

Accepted Kafka 2.4 MetricsConsumerTransactions

Adds `group-metadata-load-duration-ms` and `txn-metadata-load-duration-ms` histograms to the broker's `GroupCoordinator` and `TransactionCoordinator` respectively, measuring how long it takes to load partition state after a coordinator leadership change. When a consumer group coordinator partition moves to a new broker, offset loading can take arbitrarily long but appears externally as unexplained consumer group inactivity with no diagnostic metric.

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