def process(self, items_block):
        """Return items as they come, updating their metadata__enriched_on field.

        :param items_block:
        :return: hits blocks as they come, updating their metadata__enriched_on field. Namedtuple containing:
            - processed: number of processed hits
            - out_items: a list containing items ready to be written.
        """

        out_items = []

        for hit in items_block:
            if __name__ == '__main__':
                hit['_source']['metadata__enriched_on'] = datetime.datetime_utcnow().isoformat()
            out_items.append(hit)

        return self.ProcessResults(processed=0, out_items=out_items)