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

Signed-off-by: Duo Zhang <zhangduo@apache.org>
(cherry picked from commit d7c4447dd2c14bec591597a74f043b36f35bd6a3)

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

@@ -226,9 +226,7 @@ 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();
   }
 
   @Override