[Feature Request] Support registry metrics without the controller

[PaulSonOfLars]

We use the anka registry to store all our anka images, but do not use a controller. Due to the current layout of the exporter, this means we are unable to gather metrics on our registry, since the exporter expects to go through the controller.

It would be great if it were possible to get data straight from the registry, without going through the controller.

One possibility would be to do this by passing an ANKA_PROMETHEUS_EXPORTER_REGISTRY_ADDRESS env var/flag, and require at least one of either the registry or the controller URLs to be specified.

Thank you!