def metadata(self, item, filter_classified=False):
        """Add metadata to an item.

        It adds metadata to a given item such as how and
        when it was fetched. The contents from the original item will
        be stored under the 'data' keyword.

        :param item: an item fetched by a backend
        :param filter_classified: sets if classified fields were filtered
        """
        item = {
            'backend_name': self.__class__.__name__,
            'backend_version': self.version,
            'perceval_version': __version__,
            'timestamp': datetime_utcnow().timestamp(),
            'origin': self.origin,
            'uuid': uuid(self.origin, self.metadata_id(item)),
            'updated_on': self.metadata_updated_on(item),
            'classified_fields_filtered': self.classified_fields if filter_classified else None,
            'category': self.metadata_category(item),
            'tag': self.tag,
            'data': item,
        }

        return item
Add metadata to an item.

        It adds metadata to a given item such as how and
        when it was fetched. The contents from the original item will
        be stored under the 'data' keyword.

        :param item: an item fetched by a backend
        :param filter_classified: sets if classified fields were filtered