backup Required. The backup to update. ``backup.name``, and the
fields to be updated as specified by ``update_mask`` are
required. Other fields are ignored. Update is only supported
for the following fields:
update_mask Required. A mask specifying which fields (e.g.
``expire_time``) in the Backup resource should be updated.
This mask is relative to the Backup resource, not to the
request message. The field mask must always be specified;
this prevents any future fields from being erased
accidentally by clients that do not know about them.