KIP-403 — Increase ProducerPerformance precision by using nanoTime

Discussion Producer

Switches ProducerPerformance's latency measurement from System.currentTimeMillis() to System.nanoTime() for sub-millisecond precision. The ms-granularity timer caused many samples to report 0ms latency on fast systems, skewing percentile calculations and making it impossible to benchmark high-throughput producers accurately.

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