histograms: Refine strategies to limit the bucket count

[beorn7]

The prototype already has strategies to limit the bucket count of a Native Histograms. Vet them, and consider more strategies. For example, it might come handy to limit the overall bucket count in a HistogramVec or the overall bucket count in a registry.

This is P2 as explained in the milestone's description. Once we are confident the existing strategies are stable and we can add more later without a breaking change, this can be considered P3.