Adds a list of labels to the specified content.
        :param content_id (string): A string containing the id of the labels content container.
        :param label_names (list): A list of labels (strings) to apply to the content.
        :param callback: OPTIONAL: The callback to execute on the resulting data, before the method returns.
                         Default: None (no callback, raw data returned).
        :return: The JSON data returned from the content/{id}/label endpoint,
                 or the results of the callback. Will raise requests.HTTPError on bad input, potentially.

def create_new_label_by_content_id(self, content_id, label_names, callback=None):
        """
        Adds a list of labels to the specified content.
        :param content_id (string): A string containing the id of the labels content container.
        :param label_names (list): A list of labels (strings) to apply to the content.
        :param callback: OPTIONAL: The callback to execute on the resulting data, before the method returns.
                         Default: None (no callback, raw data returned).
        :return: The JSON data returned from the content/{id}/label endpoint,
                 or the results of the callback. Will raise requests.HTTPError on bad input, potentially.
        """
        assert isinstance(label_names, list)
        assert all(isinstance(ln, dict) and set(ln.keys()) == {"prefix", "name"} for ln in label_names)
        return self._service_post_request("rest/api/content/{id}/label".format(id=content_id),
                                         data=json.dumps(label_names), headers={"Content-Type": "application/json"},
                                         callback=callback)