OM2 - Native histograms

[jaydeluca]

• nativeHistograms:
  • Exponential bucket model: schema (-4 to 8), zero threshold/count, positive/negative spans and buckets
  • Text serialization: {count:X,sum:X,schema:N,zero_threshold:F,zero_count:X,positive_spans:[...],positive_buckets:[...]}
  • Can coexist with classic buckets on separate lines (native line MUST come first)
  • GaugeHistogram also supports native buckets (same syntax)
  • Multiple exemplars per native histogram sample
  • Spec: Native Buckets, Histogram
• No known blockers:
  • NHCB federation (#312) — still open but WG decided it won't change the spec (solution is a Prometheus scraping config, not an OM change)
  • Exemplar timestamps (#308) — closed, resolved as MUST. We're aligned