The Prometheus Exporter Bonus Pack exposes metrics for contrib modules and use-cases considered too niche for the main Prometheus Exporter module.

Contribution guide

Should my contribution be added to the Prometheus Exporter module or the Bonus Pack?

If the maintainers of the Prometheus Exporter module accept it, then that's definitely the place for it. The Bonus Pack is for more obscure or niche use-cases.