KIP-613 — Add end-to-end latency metrics to Streams

Accepted Kafka 2.6 StreamsMetrics

Introduces an `end-to-end-latency-avg` / `end-to-end-latency-max` task-level metric in Kafka Streams that measures the time from when a source record's timestamp was created (its event time) to when it is processed. There is no existing metric that captures the full pipeline latency from event time to processing time, making it difficult to bound or tune real-time application SLOs.

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