Group archives that need to not be pruned alone

## Have you checked borgbackup docs, FAQ, and open Github issues?

Yes

## Is this a BUG / ISSUE report or a QUESTION?

QUESTION

## Description:

Borg backup is a truly great product. We used it for several years.
We have one need that we would love Borg to implement.

### Context:
We host an app on multiple servers. A script is used to backup each server to a same Borg repository at roughly the same time.
So we have prefixes like so : daily_ihm, daily_api, daily_db, daily_documents.
We need all the archives of the same time available to be able to restore our app consistently.

We could create a uniq ID from the main script that will be included in the archives name. So we can identify what archives are to be restored together.

But we need to be sure that the prune will keep together matching archives.

### Proposed solution:
Being able to give the ID as an argument to borg create so archives can be matched through metadata.
Being able to tell prune to keep together archives that share the same ID. Making sure to not leave orphans archives.
If all members are to be pruned then it deletes the matching archives. If there is a conflict in the prune filtering then all the members are kept.

### Another solution:
Being able to append data from each server to the same archive.

## Question:

Is this feature something that could be implemented?
I guess we are not alone facing this need.