code: @Override
      public Map<String, Object> getPayload() {
          HashMap<String, Object> payload = new HashMap<>();
          payload.put("fun", getFunction());
          arg.ifPresent(arg -> payload.put("arg", arg));
          kwarg.ifPresent(kwarg -> payload.put("kwarg", kwarg));
          metadata.ifPresent(m -> payload.put("metadata", m));
          timeout.ifPresent(timeout -> payload.put("timeout", timeout));
          gatherJobTimeout.ifPresent(gatherJobTimeout -> payload.put("gather_job_timeout",
                  gatherJobTimeout));
          return payload;
      }
docs: {@inheritDoc}