Add exception handling for ProxyProtocolHandler and reduce log level for MQTTPreludeHandler，closes #167 (#168)

Author: ZHANGSHUNLIN

bifromq-mqtt/bifromq-mqtt-server/src/main/java/org/apache/bifromq/mqtt/handler/MQTTPreludeHandler.java

@@ -199,14 +199,14 @@ public void channelRead(ChannelHandlerContext ctx, Object msg) {
                 ctx.pipeline().remove(this);
                 break;
             default:
-                log.warn("Unsupported protocol version: {}", connectMessage.variableHeader().version());
+                log.debug("Unsupported protocol version: {}", connectMessage.variableHeader().version());
         }
     }
 
     @Override
     public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
         // simple strategy: shutdown the channel directly
-        log.warn("ctx: {}, cause:", ctx, cause);
+        log.debug("ctx: {}, cause:", ctx, cause);
         eventCollector.report(getLocal(ChannelError.class).peerAddress(remoteAddr).cause(cause));
         ctx.channel().close();
     }

bifromq-mqtt/bifromq-mqtt-server/src/main/java/org/apache/bifromq/mqtt/handler/ProxyProtocolHandler.java

@@ -24,13 +24,16 @@
 import io.netty.channel.ChannelInboundHandlerAdapter;
 import io.netty.channel.ChannelPipeline;
 import io.netty.handler.codec.haproxy.HAProxyMessage;
+import lombok.extern.slf4j.Slf4j;
+
 import java.net.InetSocketAddress;
 import java.util.Iterator;
 import java.util.Map;
 
 /**
  * This handler is used to parse the HAProxy protocol message and extract the real IP and port of the client.
  */
+@Slf4j
 public class ProxyProtocolHandler extends ChannelInboundHandlerAdapter {
 
     @Override
@@ -73,4 +76,10 @@ public void channelRead(ChannelHandlerContext ctx, Object msg) {
             ctx.pipeline().remove(this);
         }
     }
+
+    @Override
+    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
+        log.debug("ctx: {}, cause:", ctx, cause);
+    }
+
 }