Enable exporting Histogram aggregation to OTLP metric

[evantorrie]

This PR converts histogram aggregations into OTLP Histogram metrics.

I haven't changed the otlp_metric_integration_test.go yet. I believe that will require switching to use Histogram as the default aggregation for ValueRecorder as well.

[codecov]

Codecov Report

Merging #1209 into master will decrease coverage by 0.0%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1209     +/-   ##
========================================
- Coverage    76.6%   76.5%   -0.1%     
========================================
  Files         134     134             
  Lines        5760    5806     +46     
========================================
+ Hits         4414    4447     +33     
- Misses       1103    1110      +7     
- Partials      243     249      +6     

Impacted Files	Coverage Δ
exporters/otlp/internal/transform/metric.go	84.3% <ø> (-3.0%)	⬇️
sdk/export/metric/aggregation/aggregation.go	100.0% <ø> (ø)

[jmacd]

I believe that will require switching to use Histogram as the default aggregation for ValueRecorder as well.

That works for me, although it's not in the specification, neither is the current behavior.

open-telemetry/opentelemetry-specification#982 agrees.

[MrAlias]

🚀