[branch-1] getNumOpenConnections is not effective

In MetricsHBaseServerWrapperImpl

  @Override
  public int getNumOpenConnections() {
    if (!isServerStarted() || this.server.connectionList == null) {
      return 0;
    }
    return server.connectionList.size();
  }

But connectionList is created by Collections.synchronizedList(new LinkedList<Connection>());
So each connectionList.size() will compete for the mutex, we can easily replace it with another parameter numConnections, as for metrics, it is enough.