KIP-1190 — Add a metric for controller thread idleness

Accepted Kafka 4.2 KRaftMetrics

Adds an AvgIdleRatio metric to the ControllerEventManager showing the fraction of time the controller event loop thread is idle (0=always busy, 1=always idle). No metric existed for controller thread idleness, making it impossible to detect saturation, anticipate bottlenecks, or distinguish whether slow operations were caused by overloaded processing or simply rare events.

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