KIP-864 — Add End-To-End Latency Metrics to Connectors

Discussion ConnectMetrics

Adds end-to-end latency metrics for source connectors (transform-chain-source-record-time, convert-source-record-time) and sink connectors (record wall-clock lag, convert/transform time), providing a complete per-stage latency breakdown. Existing metrics only covered interaction with the external system (poll-batch-time, put-batch-latency), leaving the transformation and serialization pipeline unobservable.

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