Add additional metrics for the quota management system

We came to the conclusion that it would be really helpful to manage & operate Quay as a customer like us but also we guess it's a helpful feature for other Quay customers as well.
Especially for large deployments (4 "bridged" OpenShift clusters) as ours, it's not manageable to observe all the quotas within the UI or by notifications, cause we can't implement something like prediction.

If Quay would expose metrics for storage quotas, it would make it possible to automate the observability e.g. with Grafana.

I've metrics in mind, showing the following information about quotation:
* quota consumption per organisation:  capacity_/available_bytes
* quota consumption per repository: capacity_/available_bytes

So basically similar to kubelet_volume_stats_capacity_bytes/_available_bytes.

As this data is already available in Quay's UI (reference https://www.youtube.com/watch?v=EB3Z_2rrum4), it should be easily possible to expose the same data also via metrics and so improve the overall observability for the product.