nhcb: add global and scrape level convert_classic_histograms option

[krajorama]

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	❌	✅	❌

[kushalShukla-web]

like to work on this