`.policytagmanager.PolicyTag`
The policy tag to update. Only the description,
display_name, and parent_policy_tag fields can be updated.
update_mask
`.field_mask.FieldMask`
The update mask applies to the resource. Only display_name,
description and parent_policy_tag can be updated and thus
can be listed in the mask. If update_mask is not provided,
all allowed fields (i.e. display_name, description and
parent) will be updated. For more information including the
``FieldMask`` definition, see
https://developers.google.com/protocol-buffers/docs/reference/google.protobuf#fieldmask
If not set, defaults to all of the fields that are allowed
to update.
