Expose storage usage

[GillesMocellin]

I know that these metrics are not exposed by Nextcloud, but it would be great to have

• at least, global storage usage (even with primary storage on S3)
• better, by user usage, like that app https://apps.nextcloud.com/apps/user_usage_report

I've submitted an Issue on Nextcloud/server project nextcloud/serverinfo#464, to eventually have more info via the /ocs/v2.php/apps/serverinfo/api/v1/info API.
But perhaps nextcloud-exporter can be extended with DB queries ?

[xperimental]

@GillesMocellin Sorry for not replying, April and May have been busy so far.

I'll have a look into this soon, I hope.