KIP-410 — Add metric for request handler thread pool utilization by request type

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KIP-410 proposes adding a per-request-type JMX metric (`RequestHandlerPoolUsagePercent`) measuring what fraction of request-handler thread pool time is consumed by each API request type (Produce, Fetch, etc.). When the request handler pool is saturated, this metric lets operators identify which request type is the bottleneck, which is not distinguishable from the existing aggregate pool utilization metrics.

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