interop: add a flag to clients to statically configure grpclb (#4290)

interop/client/client.go

@@ -56,6 +56,7 @@ var (
 	defaultServiceAccount = flag.String("default_service_account", "", "Email of GCE default service account")
 	serverHost            = flag.String("server_host", "localhost", "The server host name")
 	serverPort            = flag.Int("server_port", 10000, "The server port number")
+	serviceConfigJSON     = flag.String("service_config_json", "", "Disables service config lookups and sets the provided string as the default service config.")
 	tlsServerName         = flag.String("server_host_override", "", "The server name use to verify the hostname returned by TLS handshake if it is not empty. Otherwise, --server_host is used.")
 	testCase              = flag.String("test_case", "large_unary",
 		`Configure different test cases. Valid options are:
@@ -187,6 +188,9 @@ func main() {
 			opts = append(opts, grpc.WithPerRPCCredentials(oauth.NewOauthAccess(interop.GetToken(*serviceAccountKeyFile, *oauthScope))))
 		}
 	}
+	if len(*serviceConfigJSON) > 0 {
+		opts = append(opts, grpc.WithDisableServiceConfig(), grpc.WithDefaultServiceConfig(*serviceConfigJSON))
+	}
 	opts = append(opts, grpc.WithBlock())
 	conn, err := grpc.Dial(serverAddr, opts...)
 	if err != nil {