Add "#server" as grpc.target attribute value for metrics in xDS enabled servers.

Author: DNVindhya

xds/src/main/java/io/grpc/xds/XdsServerWrapper.java

@@ -172,9 +172,7 @@ public void run() {
 
   private void internalStart() {
     try {
-      // TODO(dnvindhya): Add "#server" as "grpc.target" attribute value for
-      // xDS enabled servers.
-      xdsClientPool = xdsClientPoolFactory.getOrCreate("", new MetricRecorder() {});
+      xdsClientPool = xdsClientPoolFactory.getOrCreate("#server", new MetricRecorder() {});
     } catch (Exception e) {
       StatusException statusException = Status.UNAVAILABLE.withDescription(
               "Failed to initialize xDS").withCause(e).asException();