Add a HTTP service discovery endpoint

[znerol]

Prometheus supports several methods of dynamic target discovery. Prometheus PVE exporter might expose all (or a selection of) VMs and LXC containers in the HTTP SD format:

• Prometheus HTTP SD docs
• Proxmox VE Docs on Tags

[znerol]

There is prometheus-pve-sd. So maybe we do not need to implement that here at all.