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We should add new metrics identifying how many consumer groups are in each state.
- NumGroupsRebalancingNumGroupsPreparingRebalance: the number of consumer groups which are currently in the PreparingRebalance state.
- NumGroupsAwaitingSync: the number of consumer groups which are currently in the AwaitingSync state.
- NumGroupsStable: the number of groups which are currently in the Stable state.
- NumGroupsDead: the number of groups which are currently in the Dead state.
- NumGroupsEmpty: the number of groups which are currently in the Empty state.
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