code: public Label subscribeLabel(Object projectIdOrPath, Integer labelId) throws GitLabApiException {
          Response response = post(Response.Status.NOT_MODIFIED, getDefaultPerPageParam(),
                  "projects", getProjectIdOrPath(projectIdOrPath), "labels", labelId, "subscribe");
          return (response.readEntity(Label.class));
      }
docs: Subscribe a specified label
  
  @param projectIdOrPath the project in the form of an Integer(ID), String(path), or Project instance
  @param labelId the label ID
  @return HttpStatusCode 503
  @throws GitLabApiException if any exception occurs