Motivation for this change

I was made aware by @elseym (thanks for that 🎉) that the exporter module does not update the default value of the firewallFilter option when the port option is changed. I suspect that this was not noticed because so far no one used a custom port combined with openFirewall for an exporter.

Things done

Instead of always using the default port of one exporter for its default
firewall filter, the port from the current service configuration is used.

I did a short test to verify this works by setting a custom port for one of the exporters in the test, setting openFirewall = true; and checking the firewall rules in the test VM.

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.