KIP-964 — Have visibility when produce requests become "async"

Discussion BrokerMetrics

Proposes adding a broker metric that indicates when produce writes become synchronous (page cache dirty threshold reached), distinguishing async (memory write) from sync (disk flush) I/O paths. Operators currently have no Kafka-native signal to detect dirty-page saturation, forcing them to correlate OS-level metrics with producer timeouts after the fact.

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