KIP-434 — Add Replica Fetcher and Log Cleaner Count Metrics

Accepted Kafka 2.4 MetricsBroker

Adds JMX metrics for the current count of active replica fetcher threads and log cleaner threads on each broker. When fetcher or cleaner threads die due to unrecoverable exceptions, there is currently no metric signal—operators only notice the problem after replicas begin lagging or disks fill up.

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