numOpenConnections metric is one less than the actual

public NettyRpcServer(final Server server, final String name,
    final List<BlockingServiceAndInterface> services,
    final InetSocketAddress bindAddress, Configuration conf,
    RpcScheduler scheduler) throws IOException {
  super(server, name, services, bindAddress, conf, scheduler);
  ...
  try {
    serverChannel = bootstrap.bind(this.bindAddress).sync().channel();
    LOG.info("NettyRpcServer bind to address=" + serverChannel.localAddress()
        + ", hbase.netty.rpc.server.worker.count=" + workerCount
        + ", useEpoll=" + useEpoll);
    allChannels.add(serverChannel);
  } catch (InterruptedException e) {
    throw new InterruptedIOException(e.getMessage());
  }
  ...
}

@Override
public int getNumOpenConnections() {
  // allChannels also contains the server channel, so exclude that from the count.
  return allChannels.size() - 1;
}

public NettyRpcServer(Server server, String name, List<BlockingServiceAndInterface> services,
    InetSocketAddress bindAddress, Configuration conf, RpcScheduler scheduler)
    throws IOException {
  ...
  try {
    serverChannel = bootstrap.bind(this.bindAddress).sync().channel();
    LOG.info("NettyRpcServer bind to address=" + serverChannel.localAddress());
  } catch (InterruptedException e) {
    throw new InterruptedIOException(e.getMessage());
  }
  ...
}