private RaftGroup buildRaftGroup(ConsensusGroupId groupId, List<Peer> peers) {
    return RaftGroup.valueOf(
        Utils.fromConsensusGroupIdToRaftGroupId(groupId),
        Utils.fromPeersAndPriorityToRaftPeers(peers, DEFAULT_PRIORITY));
  }
returns the known leader to the given group. NOTICE: if the local peer isn't a member of given
group, getLeader will return null.

@return null if local peer isn't in group, otherwise group leader.
iotdb-core/consensus/src/main/java/org/apache/iotdb/consensus/ratis/RatisConsensus.java