private RatisClient getRaftClient(RaftGroup group) throws ClientManagerException {
    try {
      return clientManager.borrowClient(group);
    } catch (ClientManagerException e) {
      logger.error("Borrow client from pool for group {} failed.", group, e);
      // rethrow the exception
      throw e;
    }
  }
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