KIP-1286 — Add last-applied-image-latency metric for brokers

Discussion KRaftMetrics

Adds a last-applied-image-latency-ms metric to brokers that measures the time between the latest cluster metadata image being committed by the controller and being applied by the broker. The existing last-applied-record-lag-ms metric compares broker clock vs. record timestamp, which conflates broker clock skew with actual metadata replication lag, making it unreliable for detecting slow metadata propagation.

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