KIP-1207 — Fix anomaly of JMX metrics RequestHandlerAvgIdlePercent in kraft combined mode

Accepted Kafka 4.2 KRaftMetrics

Fixes the RequestHandlerAvgIdlePercent JMX metric in KRaft combined mode (broker+controller co-located) where the value incorrectly reported ~2.0 instead of 0–1, and adds separate BrokerRequestHandlerAvgIdlePercent and ControllerRequestHandlerAvgIdlePercent metrics. In combined mode, two separate KafkaRequestHandlerPool objects both divided their idle time by numIoThreads before reporting to a shared Meter, effectively halving the denominator and doubling the reported ratio.

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