We previously filtered out the `_module` attribute in a NixOS
configuration by filtering it using the option's `apply` function.

This meant that every option that had a submodule type needed to have
this apply function. Adding this function is easy to forget thus this
mechanism is error prone.

We now recursively filter out the `_module` attributes at the place we
construct the Prometheus configuration file. Since we now do the filtering
centrally we don't have to do it per option making it less prone to errors.

This makes sure that when a user hasn't set a Prometheus option it
won't show up in the prometheus.yml configuration file. This results
in smaller and easier to understand configuration files.

nixos/prometheus: refactored & added more missing options