interop-testing: use server hostname instead of id for xds test (#6639)

interop-testing/src/main/java/io/grpc/testing/integration/XdsTestClient.java

@@ -208,17 +208,17 @@ public final class XdsTestClient {
                 CallOptions.DEFAULT.withDeadlineAfter(rpcTimeoutSec, TimeUnit.SECONDS));
         call.start(
             new ClientCall.Listener<SimpleResponse>() {
-              private String serverId;
+              private String hostname;
 
               @Override
               public void onMessage(SimpleResponse response) {
-                serverId = response.getServerId();
+                hostname = response.getHostname();
                 // TODO(ericgribkoff) Currently some test environments cannot access the stats RPC
                 // service and rely on parsing stdout.
                 if (printResponse) {
                   System.out.println(
                       "Greeting: Hello world, this is "
-                          + response.getHostname()
+                          + hostname
                           + ", from "
                           + call.getAttributes().get(Grpc.TRANSPORT_ATTR_REMOTE_ADDR));
                 }
@@ -227,7 +227,7 @@ public final class XdsTestClient {
               @Override
               public void onClose(Status status, Metadata trailers) {
                 for (XdsStatsWatcher watcher : savedWatchers) {
-                  watcher.rpcCompleted(requestId, serverId);
+                  watcher.rpcCompleted(requestId, hostname);
                 }
               }
             },
@@ -295,14 +295,14 @@ public final class XdsTestClient {
       this.endId = endId;
     }
 
-    void rpcCompleted(long requestId, @Nullable String serverId) {
+    void rpcCompleted(long requestId, @Nullable String hostname) {
       synchronized (lock) {
         if (startId <= requestId && requestId < endId) {
-          if (serverId != null) {
-            if (rpcsByPeer.containsKey(serverId)) {
-              rpcsByPeer.put(serverId, rpcsByPeer.get(serverId) + 1);
+          if (hostname != null) {
+            if (rpcsByPeer.containsKey(hostname)) {
+              rpcsByPeer.put(hostname, rpcsByPeer.get(hostname) + 1);
             } else {
-              rpcsByPeer.put(serverId, 1);
+              rpcsByPeer.put(hostname, 1);
             }
           } else {
             noRemotePeer += 1;

interop-testing/src/main/java/io/grpc/testing/integration/XdsTestServer.java

@@ -124,13 +124,14 @@ public final class XdsTestServer {
   }
 
   private class TestServiceImpl extends TestServiceGrpc.TestServiceImplBase {
-    private String host = "";
+    private final String host;
 
     private TestServiceImpl() {
       try {
         host = InetAddress.getLocalHost().getHostName();
       } catch (UnknownHostException e) {
-        logger.log(Level.WARNING, "Failed to get host", e);
+        logger.log(Level.SEVERE, "Failed to get host", e);
+        throw new RuntimeException(e);
       }
     }