HDFS-15974. RBF: Unable to display the datanode UI of the router (#2915)

Co-authored-by: zhuxiangyi <zhuxiangyi@360.cn>
Reviewed-by: Inigo Goiri <inigoiri@apache.org>
Signed-off-by: Takanobu Asanuma <tasanuma@apache.org>
(cherry picked from commit dff95c5eca1c0d9a43b5734f93320fa51aee63da)

hadoop-hdfs-project/hadoop-hdfs-rbf/src/main/java/org/apache/hadoop/hdfs/server/federation/metrics/NamenodeBeanMetrics.java

@@ -825,7 +825,7 @@ public class NamenodeBeanMetrics
 
   @Override
   public String getEnteringMaintenanceNodes() {
-    return "N/A";
+    return "{}";
   }
 
   @Override

hadoop-hdfs-project/hadoop-hdfs-rbf/src/test/java/org/apache/hadoop/hdfs/server/federation/router/TestRouterRpc.java

@@ -1717,6 +1717,14 @@ public class TestRouterRpc {
     assertEquals(0, metrics.getNumLiveNodes());
   }
 
+  @Test
+  public void testNamenodeMetricsEnteringMaintenanceNodes() throws IOException {
+    final NamenodeBeanMetrics metrics =
+            router.getRouter().getNamenodeMetrics();
+
+    assertEquals("{}", metrics.getEnteringMaintenanceNodes());
+  }
+
   @Test
   public void testCacheAdmin() throws Exception {
     DistributedFileSystem routerDFS = (DistributedFileSystem) routerFS;