KIP-180 — Add a broker metric specifying the number of consumer group rebalances in progress

Accepted Kafka 1.1 ConsumerMetrics

KIP-180 adds JMX metrics counting the number of consumer groups in each coordinator state (`PreparingRebalance`, `CompletingRebalance`, `Stable`, `Dead`, `Empty`) and renames the confusingly named `AwaitingSync` state to `CompletingRebalance`. Without per-state metrics, operators cannot distinguish between a cluster that is continuously rebalancing and one that is stable, making it impossible to set meaningful rebalance-rate alerts.

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