KIP-517 — Add consumer metrics to observe user poll behavior

Accepted Kafka 2.4 ConsumerMetrics

Adds consumer metrics including `poll-idle-ratio`, `time-between-poll-avg`, `time-between-poll-max`, and `last-poll-seconds-ago` to expose how frequently and how promptly the application calls `Consumer.poll()`. Without these metrics, there is no observable signal to distinguish whether a `max.poll.interval.ms` violation is caused by slow record processing or by the application neglecting to call `poll()` frequently enough.

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