Add more Prometheus metrics

Is your feature request related to a problem? Please describe.
We want to monitor our Harbor instances with Prometheus, but there are some metrics missing for some checks.

• Currently, there is no metric with the status of Registries healthy/unhealthy (in our case used for Proxy Cache Projects).
• Storage consumption of Local Disks (like in the /api/v2.0/systeminfo/volumes API endpoint)

Describe the solution you'd like
A new Metric under the /metrics endpoint with the health status of the Registries.

Example of additional required metrics

harbor_registries_healthy{registry="dockerhub"} 1
harbor_registries_healthy{registry="broken-registry"} 0

harbor_disk_bytes_free 186311114752
harbor_disk_bytes_total 1099511627776