code: public static Authenticator create() {
      ServiceConfigFetcher fetcher = ServiceConfigFetcher.create();
      Service service = fetcher.fetch();
      if (!service.hasAuthentication()) {
        throw new IllegalArgumentException("Authentication is not defined in service config");
      }
      return create(service.getAuthentication(), Clock.SYSTEM);
    }
docs: Create an instance of {@link Authenticator} using the service configuration fetched from Google
  Service Management APIs.
  
  @return an {@code Authenticator}
  @throws java.lang.IllegalArgumentException if the authentication message is not defined in the
  fetched service config.