Native histograms: testutil to provide ways to verify native histograms #1824
Description
Proposal
Related to grafana/mimir#11573 where someone is adding a pure native histogram.
Currently (June 2025) there's no text representation implemented for native histograms so one cannot use the GatherAndCompare from the testutil package to assert on: existence and value of native histograms.
The text format for native histograms - as proposed currently - isn't straightforward for developers to assert on. Doesn't have buckets, rather it has spans and deltas and such.
A solution is needed to be able to:
- check that a native histogram was gathered
- check the count
- check the sum
- check that it has expected number of observations in an interval