KIP-490 — New metric to count offsets expired without being consumed by a consumer group

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KIP-490 proposes a topic-level config `non.consumed.offsets.groups` listing consumer groups for which the broker emits a JMX metric counting messages that have been deleted by retention before being consumed by the group. Without this metric, consumers have no signal that their offset has been overtaken by the log-start-offset, meaning messages were silently lost due to retention.

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