HDFS-16065. RBF: Add metrics to record Router's operations (#3100)

Author: symious

hadoop-hdfs-project/hadoop-hdfs-rbf/src/main/java/org/apache/hadoop/hdfs/server/federation/router/Router.java

@@ -643,6 +643,18 @@ public RouterMetrics getRouterMetrics() {
     return null;
   }
 
+  /**
+   * Get the metrics system for the Router Client.
+   *
+   * @return Router Client metrics.
+   */
+  public RouterClientMetrics getRouterClientMetrics() {
+    if (this.metrics != null) {
+      return this.metrics.getRouterClientMetrics();
+    }
+    return null;
+  }
+
   /**
    * Get the federation metrics.
    *