void recordNewServerWithLock(final ServerName serverName, final ServerMetrics sl) {
    LOG.info("Registering regionserver=" + serverName);
    this.onlineServers.put(serverName, sl);
    master.getAssignmentManager().getRegionStates().createServer(serverName);
  }
Adds the onlineServers list. onlineServers should be locked.
@param serverName The remote servers name.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/ServerManager.java