KIP-1131 — Improved controller-side monitoring of broker states

Accepted Kafka 4.1 KRaftMetrics

Adds per-broker `BrokerRegistrationState` metrics and a `ControlledShutdownBrokerCount` metric on the KRaft controller to expose individual broker states without requiring log inspection. The existing `ActiveControllerCount` and fenced/unfenced broker count metrics provide only aggregate totals; diagnosing which specific broker entered a particular state requires manual parsing of the metadata log.

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