- [ ] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [ ] I have checked the [FAQ](https://github.com/apache/dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 2.7.3
* Operating System version: macOs sierra
* Java version: 1.8

### Steps to reproduce this issue

Sometimes, when debugging connect time issue, we don't know whether the port is listening.  We can check the connected log if the connect timeout occurs.
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelPromise;
import io.netty.handler.timeout.IdleStateEvent;
/**
 * NettyClientHandler
 */
@io.netty.channel.ChannelHandler.Sharable
public class NettyClientHandler extends ChannelDuplexHandler {
    private static final Logger logger = LoggerFactory.getLogger(NettyClientHandler.class);
    private final URL url;
    private final ChannelHandler handler;
    public NettyClientHandler(URL url, ChannelHandler handler) {
        if (url == null) {
            throw new IllegalArgumentException("url == null");
        }
        if (handler == null) {
            throw new IllegalArgumentException("handler == null");
        }
        this.url = url;
        this.handler = handler;
    }
    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        NettyChannel channel = NettyChannel.getOrAddChannel(ctx.channel(), url, handler);
        try {
            handler.connected(channel);
        } finally {
            NettyChannel.removeChannelIfDisconnected(ctx.channel());
        }
    }