code: public Optional<Label> createLabel(String monitorId, Label label)
      {
          HTTP.POST(String.format("/v1/monitors/%s/labels", monitorId), label.getKey());
          return Optional.of(label);
      }
docs: Adds the given label to the monitor with the given id.
  @param monitorId The id of the monitor to update
  @param label The label to add
  @return The label that was added