Export more metrics via ocs/v2.php/apps/serverinfo/api/v1/info

[GillesMocellin]

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Is your feature request related to a problem? Please describe.
I want to have a general overview of Nextcloud usage via Grafana/Prometheus + nextcloud-exporter.
But the data coming from the API /ocs/v2.php/apps/serverinfo/api/v1/info lak at least one very useful information : the storage used, even if primary storage is on an external S3 system.
I don't know if it's possible to go further and add storage by user (perhaps not in that general API but associated with the app https://apps.nextcloud.com/apps/user_usage_report)
Perhaps that's what should be in the actual num_storages_other metric ?

Describe the solution you'd like
I'd like to see global storage usage in /ocs/v2.php/apps/serverinfo/api/v1/info, not only for local storage.

Describe alternatives you've considered
I'm in the process of trying an SQL query Prometheus exporter (https://github.com/albertodonato/query-exporter), to see if I can get total and user usage in my Grafana Dashboard.