Add Exemplar to metricdata package

[MrAlias]

Part of #3834

• Add the new Exemplar[N int64 | float64] struct to sdk/metric/metricdata
• Add Exemplars []Exemplar[N] field to DataPoint in sdk/metric/metricdata
• Add Exemplars []Exemplar[N] field to HistogramDataPoint in sdk/metric/metricdata
  • Update HistogramDataPoint to be defined generically as HistogramDataPoint[N int64 | float64]
  • Update Histogram to be defined generically as Histogram[N int64 | float64]
• Update metricdatatest to support Exemplar
• Update metricdatatest to support HistogramDataPoint[N]
• Update sdk/metric histogram aggregator to produce Histogram[N]
• Update exporters (stdout, Prometheus, OTLP) to compile with Histogram[N]
  • These exporters still need to be updated to export Exemplar
• Update OpenCensus bridge to support Histogram[N] (only produced Histogram[float64])

[codecov]

Codecov Report

Merging #3849 (8e6506a) into main (b62eb2c) will decrease coverage by 0.2%.
The diff coverage is 72.6%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3849     +/-   ##
=======================================
- Coverage   81.7%   81.6%   -0.2%     
=======================================
  Files        169     169             
  Lines      12581   12723    +142     
=======================================
+ Hits       10288   10387     +99     
- Misses      2078    2117     +39     
- Partials     215     219      +4     

Impacted Files	Coverage Δ
sdk/metric/metricdata/data.go	57.1% <0.0%> (ø)
exporters/stdout/stdoutmetric/exporter.go	92.7% <50.0%> (-4.5%)	⬇️
...dk/metric/metricdata/metricdatatest/comparisons.go	88.2% <62.7%> (-9.4%)	⬇️
bridge/opencensus/internal/ocmetric/metric.go	93.3% <100.0%> (ø)
...s/otlp/otlpmetric/internal/transform/metricdata.go	100.0% <100.0%> (ø)
exporters/prometheus/exporter.go	83.7% <100.0%> (+0.1%)	⬆️
sdk/metric/internal/histogram.go	100.0% <100.0%> (ø)
sdk/metric/metricdata/metricdatatest/assertion.go	87.0% <100.0%> (+2.9%)	⬆️

... and 2 files with indirect coverage changes

[pellared]

General question, just to clarify: do we want to implement "Exemplar" before it is Stable in the specification?

Reference: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#exemplar

[MrAlias]

General question, just to clarify: do we want to implement "Exemplar" before it is Stable in the specification?

Reference: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk.md#exemplar

I'm not sure we want to implement it given the unstable nature of the specification around it. That and I think the specification has some evolving to do regarding the exemplar.

That said, I do want to make sure we can add support in the future. These changes are necessary now so we don't have to implement something sub-par to support them in the future or introduce backwards incompatible changes later.

As pointed out in your other comment, I think there is still some follow-up work to address here.

1. The other fields of the HistogramDataPoint can now utilize the generic argument to give native values
2. The exporters can be updated to "support" exemplars as they will be in the data structure passed to them (though the SDK will not produce them yet)

I think (2) is helpful on the path to supporting exemplars, but not needed for the GA of the specification.