Migrate to custom collector class

[mweinelt]

https://prometheus.github.io/client_python/collector/custom/

Recommendation from people on #prometheus on libera

https://github.com/tykling/dns_exporter could be a good example of how it can be used.

[Qwiko]

I am looking into this at the moment. I believe this would solve a lot of issues and workarounds that exist in the current code.
According to this: https://prometheus.io/docs/instrumenting/writing_exporters the best practise is to create new metrics on every fetch. That means for kea-exporter a complete rebuild of the code to create the metrics dynamically instead of populating static metrics with data from statistics.

I will keep on working and provide some code for others to give feedback on.