Use nixos-fw chain instead of INPUT so that the rules don't keep
stacking everytime the firewall is reloaded.
This also adds a comment to each rule about the associated exporter.
- prometheus exporters are now configured with
`services.prometheus.exporters.<name>`
- the exporters are now defined by attribute sets
from which the options for each exporter are generated
- most of the exporter definitions are used unchanged,
except for some changes that should't have any impact
on the functionality.
