Add alternative Haskell client library

[brandonchinn178]

prometheus is another popular client library for Prometheus; this documentation should probably not take a stance on which one is recommended

[jan--f]

Hello from the bug scrub! Thank you for your contribution.

Iiuc the two hackage packages are generated from https://github.com/fimad/prometheus-haskell. So this doesn't add anything unless I'm mistaken. By convention I would prefer to keep links to the source code over language packages.

Regarding the multilevel list: I'm not against this but it departs from a currently consistent style. If you would like to propose introducing this, please open a PR for the purpose and also consider other similar list (like https://prometheus.io/docs/instrumenting/exporters/ and possible others) and add a string argument why multilevel lists are an improvement.

In the mean time I'll close this PR here. Please feel free to reopen if you disagree with any of the above.

[brandonchinn178]

@jan--f No, these are two completely separate libraries. prometheus is not generated from prometheus-client

[brandonchinn178]

I'm unable to reopen the PR. Can you please reopen it?