KIP-1314 — Support broker-side consumer group rebalance callback

Discussion BrokerConsumerMetrics

Adds a broker-side extension interface invoked by the GroupCoordinator when a consumer group's epoch is bumped, plus a broker config to install it, exposing rebalance context including the group id. Client-side rebalance listeners depend on SDK adoption and can be silenced or logged away, so incident diagnosis falls back to parsing coordinator logs, while existing broker metrics omit the group id and would explode in cardinality if they carried it.

Details