KIP-1344 — Add Transaction Coordinator Metrics for Transaction State Log Appends and Marker Completion

Discussion TransactionsMetricsBroker

Adds transaction coordinator metrics covering the durable __transaction_state appends for PREPARE_COMMIT/PREPARE_ABORT and COMPLETE_COMMIT/COMPLETE_ABORT transitions and the number of transactions currently waiting for marker completion. Existing request latency metrics show that EndTxn is slow without separating coordinator validation, queueing, metadata locking, and the transaction state log append, and generic storage/replication metrics cannot attribute pressure to the transaction state log, so operators cannot tell whether latency comes from the prepare transition, marker fan-out to partition leaders, or the final log append after the client already got a success response.

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