KIP-472 — [STREAMS] Add partition time field to RecordContext

Discarded Kafka 2.4 Streams

Proposes adding a `partitionTime()` accessor to `RecordContext` in Kafka Streams so processors can read the current partition time (stream-time high-watermark for that partition) alongside the individual record timestamp (discarded as too complex for the marginal benefit). When a filter processor drops the highest-timestamp records, downstream tasks inherit an incorrect (too-high) partition time that causes premature window and grace period advancement.

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