nixos/prometheus: add mqtt-exporter

[christoph-heiss]

Description of changes

This adds mqtt-exporter, a MQTT exporter for Prometheus, as a package itself.
Adds a corresponding services.prometheus.exporters.mqtt exporter, such that it can be easily configured & enabled. As well as a test for it, as it should be.

Depends on #288219.

Obsoleted by 1.4.2

Two patches are unfortunaly (currently) needed, one for providing some basic pyproject properties to allow it to build successfully, as upstream does not have [project] defined in their pyproject.toml, as well as not have setup.{cfg,py}. We could also manually assemble the package I guess, but that seemed like a pretty clean solution to me - or there is some even better solution?

Second patch addresses the lack of a configuration option to set the port the metrics exporter should listen on, which is an common option pre-defined for all exporters.

I intend to upstream both patches - in fact, I have already sent a PR upstream with both of them, so hopefully they can be dropped soon.

Both changes have been merged upstream, so they can be dropped with the next release. 🚀

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

---

Add a 👍 reaction to pull requests you find important.

[christoph-heiss]

Ping :^)

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/prs-ready-for-review/3032/4627

[FliegendeWurst]

LGTM

[christoph-heiss]

LGTM

Thanks for the review! Fixed your suggestion.

[christoph-heiss]

I've now also added a check phase using pytestCheckHook, FWIW.

[FliegendeWurst]

Would be good to have a prometheus maintainer review

[christoph-heiss]

Package has been added in #360486 separately ..