nhcb: write basic unit tests for custom histograms and implement

[krajorama]

Proposal

Depends on #13508

Add unit tests for histogram_avg, histogram_count, histogram_sum,.. functions on custom histograms
Add unit tests for operations on custom histograms that share the same bucket layout. Addition, average, max, min, etc.

[miledxz]

Heey @krajorama, feel free to assign me on this one,
would be good to add required tests

[krajorama]

@zedGGs thank you for the offer. FYI, there's a bunch of dependencies to be able to start testing this for real, follow the links in the description. If you still feel like starting, please target the https://github.com/prometheus/prometheus/tree/nhcb branch with PRs.