code: @Override
      protected RpcResponse onFailure(ServiceRequestContext ctx, RpcRequest req, @Nullable Throwable cause)
              throws Exception {
          return RpcResponse.ofFailure(HttpStatusException.of(HttpStatus.SERVICE_UNAVAILABLE));
      }
docs: Invoked when {@code req} is throttled. By default, this method responds with a
  {@link HttpStatusException} with {@code 503 Service Unavailable}.