KIP-121 — Add KStream peek method

Accepted Streams

Adds a peek(ForeachAction) method to KStream that applies a side-effect action (e.g., logging, metrics increment) to each record and passes the record unchanged to the next operator. KStream lacked a non-terminal inspection point; developers had to use map() with an identity transformation to observe records mid-pipeline, which could trigger unnecessary repartitioning.

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