KIP-1001 — Add CurrentControllerId Metric

Accepted Kafka 3.7 KRaftMetrics

Adds a `CurrentControllerId` metric exposed on every broker and controller node that reports the node ID of the active KRaft (or ZooKeeper) controller, or -1 if unknown. Identifying the current active controller requires either checking each node's `ActiveControllerCount` metric individually or parsing metadata logs; there is no single metric queryable from any node that identifies which node is the controller.

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