Storing arbitrary metadata in a backup for use in other tooling

There are some operations on backups that require a connection to the cluster the backup is for. One example of this is expiring data by timestamp. If someone is building tooling that is going to operate on lots of backups for lots of different clusters, they need a way to determine which cluster is using which backup. Right now the only metadata we have to make this kind of determination is the backup name. This may not be enough information for someone whose system has a multi-level hierarchy that they use to manage clusters. As an alternative, could we provide a mechanism to provide arbitrary metadata when starting a backup and surfacing that metadata through the backup describe command?