Start up & discover coordinator:Consumer检测其所在Group的Coordinator。一旦它检测到Coordinator,即向其发送JoinGroupRequest。
Part of a group:该状态下,Consumer已经是该Group的成员,并周期性发送HeartbeatRequest。如HeartbeatResponse包含IllegalGeneration错误码,则转换到Stopped Consumption状态。若连接丢失,HeartbeatResponse包含NotCoordinatorForGroup错误码,则转换到Rediscover coordinator状态。