nixos/prometheus: filter out the _module attr in a central place

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.

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