static String classicGroupHeartbeatKey(String groupId, String memberId) {
        return "heartbeat-" + groupId + "-" + memberId;
    }
Generate a classic group heartbeat key for the timer.

Package private for testing.

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

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