var google.golang.org/grpc/internal/envconfig.C2PResolverTestOnlyTrafficDirectorURI

one use

	google.golang.org/grpc/internal/envconfig (current package)
		xds.go#L91: 	C2PResolverTestOnlyTrafficDirectorURI = os.Getenv("GRPC_TEST_ONLY_GOOGLE_C2P_RESOLVER_TRAFFIC_DIRECTOR_URI")