code: public ServiceCall router(Class<? extends Router> routerType) {
      ServiceCall target = new ServiceCall(this);
      target.router = Routers.getRouter(routerType);
      return target;
    }
docs: Creates new {@link ServiceCall}'s definition with a given router.
  
  @param routerType given class of the router.
  @return new {@link ServiceCall} instance.