[xDS e2e tests] fix flake in metric test (#37653)

Closes #37653

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37653 from markdroth:xds_e2e_metric_test_flake 57dad466b7
PiperOrigin-RevId: 672556528

test/cpp/end2end/xds/xds_core_end2end_test.cc

@@ -1244,7 +1244,10 @@ TEST_P(XdsMetricsTest, MetricValues) {
   EdsResourceArgs args =
       EdsResourceArgs({{"locality0", CreateEndpointsForBackends()}});
   balancer_->ads_service()->SetEdsResource(BuildEdsResource(args));
-  CheckRpcSendOk(DEBUG_LOCATION);
+  // Use wait_for_ready and increase timeout, in case the client takes a
+  // little while to get connected.
+  CheckRpcSendOk(DEBUG_LOCATION, /*times=*/1,
+                 RpcOptions().set_wait_for_ready(true).set_timeout_ms(15000));
   stats_plugin_->TriggerCallbacks();
   // Check client metrics.
   EXPECT_THAT(stats_plugin_->GetInt64CallbackGaugeValue(