nhcb: add global and scrape level convert_classic_histograms option #13532
Description
Proposal
Depends on #13486 #13487 #13529 at least :)
The configuration option should enable conversion upon scrape.
Documentation is also included in this task.
Configuration and expected outcome
In all cases the native histograms feature is enabled. When the feature is disabled, there are no native histograms, so classic histograms have to be produced by the scrape.
Case 1. Metric instrumentation only exposes classic histograms
Typical legacy application scenario.
| config | classic float series | exponential nh | custom bucket nh |
|---|---|---|---|
| scrape_classic=false, convert_classic=false | ✅ | ❌ | ❌ |
| scrape_classic=true, convert_classic=false | ✅ | ❌ | ❌ |
| scrape_classic=false, convert_classic=true | ❌ | ❌ | ✅ |
| scrape_classic=true, convert_classic=true | ✅ | ❌ | ✅ |
Case 2. Metric instrumentation exposes both classic and exponential native histograms
Exponential native histogram has more information, preferred over custom buckets. Note: we must make a choice since both custom bucket and exponential histogram has the same metric name.
| config | classic float series | exponential nh | custom bucket nh |
|---|---|---|---|
| scrape_classic=false, convert_classic=false | ❌ | ✅ | ❌ |
| scrape_classic=true, convert_classic=false | ✅ | ✅ | ❌ |
| scrape_classic=false, convert_classic=true | ❌ | ✅ | ❌ |
| scrape_classic=true, convert_classic=true | ✅ | ✅ | ❌ |
Case 3. Metric instrumentation only exposes exponential native histograms
No classic histogram to expose or convert.
| config | classic float series | exponential nh | custom bucket nh |
|---|---|---|---|
| scrape_classic=false, convert_classic=false | ❌ | ✅ | ❌ |
| scrape_classic=true, convert_classic=false | ❌ | ✅ | ❌ |
| scrape_classic=false, convert_classic=true | ❌ | ✅ | ❌ |
| scrape_classic=true, convert_classic=true | ❌ | ✅ | ❌ |