Determine the metadata path.

def metadata_path(self):
        """Determine the metadata path."""
        xml_name = _granule_identifier_to_xml_name(self.granule_identifier)
        metadata_path = os.path.join(self.granule_path, xml_name)
        try:
            assert os.path.isfile(metadata_path) or \
                metadata_path in self.dataset._zipfile.namelist()
        except AssertionError:
            raise S2ReaderIOError(
                "Granule metadata XML does not exist:", metadata_path)
        return metadata_path