Enable xds retry test for cpp (#26828)

* enable retry

* enable cpp

* fix cpp format

* fix indent

test/cpp/interop/xds_interop_client.cc

@@ -465,12 +465,15 @@ class XdsUpdateClientConfigureServiceImpl
 void RunTestLoop(std::chrono::duration<double> duration_per_query,
                  StatsWatchers* stats_watchers,
                  RpcConfigurationsQueue* rpc_configs_queue) {
+  grpc::ChannelArguments channel_args;
+  channel_args.SetInt(GRPC_ARG_ENABLE_RETRIES, 1);
   TestClient client(
-      grpc::CreateChannel(absl::GetFlag(FLAGS_server),
+      grpc::CreateCustomChannel(absl::GetFlag(FLAGS_server),
                                 absl::GetFlag(FLAGS_secure_mode)
                                     ? grpc::experimental::XdsCredentials(
                                           grpc::InsecureChannelCredentials())
-                              : grpc::InsecureChannelCredentials()),
+                                    : grpc::InsecureChannelCredentials(),
+                                channel_args),
       stats_watchers);
   std::chrono::time_point<std::chrono::system_clock> start =
       std::chrono::system_clock::now();

tools/run_tests/xds_k8s_test_driver/tests/url_map/retry_test.py

@@ -63,8 +63,9 @@ def _build_retry_route_rule(retryConditions, num_retries):
     }
 
 
-@absltest.skipUnless('java-client' in xds_k8s_flags.CLIENT_IMAGE.value,
-                     'Retry is currently only implemented in Java.')
+@absltest.skipUnless('cpp-client' in xds_k8s_flags.CLIENT_IMAGE.value or \
+                     'java-client' in xds_k8s_flags.CLIENT_IMAGE.value,
+                     'Xds-retry is currently only implemented in C++ and Java.')
 class TestRetryUpTo3AttemptsAndFail(xds_url_map_testcase.XdsUrlMapTestCase):
 
     @staticmethod
@@ -100,8 +101,9 @@ class TestRetryUpTo3AttemptsAndFail(xds_url_map_testcase.XdsUrlMapTestCase):
                                  tolerance=_NON_RANDOM_ERROR_TOLERANCE)
 
 
-@absltest.skipUnless('java-client' in xds_k8s_flags.CLIENT_IMAGE.value,
-                     'Retry is currently only implemented in Java.')
+@absltest.skipUnless('cpp-client' in xds_k8s_flags.CLIENT_IMAGE.value or \
+                     'java-client' in xds_k8s_flags.CLIENT_IMAGE.value,
+                     'Xds-retry is currently only implemented in C++ Java.')
 class TestRetryUpTo4AttemptsAndSucceed(xds_url_map_testcase.XdsUrlMapTestCase):
 
     @staticmethod