KIP-1229 — Add Idle Thread Ratio Metric to MetadataLoader

Accepted Kafka 4.2 KRaftMetrics

Adds an AvgIdleRatio metric to the MetadataLoader event queue thread, mirroring the existing metric on QuorumController's ControllerEventManager. The MetadataLoader, which distributes metadata changes from the Raft layer to publishers like the BrokerMetadataListener, lacked any idle-ratio visibility, making it impossible to detect metadata propagation bottlenecks in KRaft clusters.

Details