Restoring metadata only needs to be done once

metadata is full backup. Only the last backup need to restore metadata.
cinder/backup/chunkeddriver.py:691
        while index >= 0:
            backup1 = backup_list[index]
            index = index - 1
            metadata = self._read_metadata(backup1)
            restore_func(backup1, volume_id, metadata, volume_file)

            volume_meta = metadata.get('volume_meta', None)
            try:
                if volume_meta:
                    self.put_metadata(volume_id, volume_meta)
                else:
                    LOG.debug("No volume metadata in this backup.")
            except exception.BackupMetadataUnsupportedVersion:
                msg = _("Metadata restore failed due to incompatible version.")
                LOG.error(msg)
                raise exception.BackupOperationError(msg)