HBASE-27379 fix numOpenConnections metric is one less than the actual (#4884)

Signed-off-by: Duo Zhang <zhangduo@apache.org>

hbase-server/src/main/java/org/apache/hadoop/hbase/ipc/NettyRpcServer.java

@@ -232,9 +232,7 @@ public class NettyRpcServer extends RpcServer {
 
   @Override
   public int getNumOpenConnections() {
-    int channelsCount = allChannels.size();
-    // allChannels also contains the server channel, so exclude that from the count.
-    return channelsCount > 0 ? channelsCount - 1 : channelsCount;
+    return allChannels.size();
   }
 
   private void initSSL(ChannelPipeline p, boolean supportPlaintext)