fix: specify Metadata-Flavor for metadata requests (#1397)

Author: Erik

google-api-client/src/main/java/com/google/api/client/googleapis/auth/oauth2/OAuth2Utils.java

@@ -83,6 +83,7 @@ static boolean runningOnComputeEngine(HttpTransport transport,
       try {
         HttpRequest request = transport.createRequestFactory().buildGetRequest(tokenUrl);
         request.setConnectTimeout(COMPUTE_PING_CONNECTION_TIMEOUT_MS);
+        request.getHeaders().set("Metadata-Flavor", "Google");
         HttpResponse response = request.execute();
         try {
           HttpHeaders headers = response.getHeaders();