To fulfill one time backup requests there needs to be a new controller that reconciles an EtcdBackup CustomResource (CR) object and executes and saves a one time backup of the etcd cluster.
 
Similar to the upgradebackupcontroller the controller would be triggered to create a backup pod/job which would save the backup to the PersistentVolume specified by the spec of the EtcdBackup CR object.

The controller would also need to honor the retention policy specified by the EtcdBackup spec and update the status accordingly.

See the following enhancement and API PRs for more details and potential updates to the API and workflow for the one time backup:
https://github.com/openshift/enhancements/pull/1370
https://github.com/openshift/api/pull/1482