static String consumerGroupSyncKey(String groupId, String memberId) {
        return "sync-" + groupId + "-" + memberId;
    }
Generate a consumer group sync key for the timer.

Package private for testing.

@param groupId   The group id.
@param memberId  The member id.

@return the sync key.
group-coordinator/src/main/java/org/apache/kafka/coordinator/group/GroupMetadataManager.java