KIP-736 — Report the true end to end fetch latency

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Adds per-follower latency breakdown metrics to the fetch purgatory so operators can see how long fetch requests spend waiting for each follower to acknowledge, decomposing end-to-end produce latency more precisely. The existing `RemoteTime` metric for produce requests aggregates all follower wait time without distinguishing between network congestion and slow fetch processing per follower.

Protocol Impact

Fetch

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