OpenCensus metric exporter bridge

[dashpole]

Fixes #93

This allows wrapping an OpenTelemetry exporter so that it can be used with OpenCensus. This is useful to facilitate migrations from OpenCensus to OpenTelemetry, as it would allow using a mix of OpenCensus and OpenTelemetry APIs in code (and libraries), while sending all telemetry to a single exporter.

Everything was straightforward to map, except the data points themselves. To do that, I ended up having to implement custom Aggregations based on the data point type.

I also need to do an audit of OpenCensus functionality that is "dropped" when using the bridge and document it. Thus far, I know it drops Summary data points, exemplars, and some types of metric descriptors. I might do that in a follow-up.

[codecov]

Codecov Report

Merging #1444 (93877af) into main (77aa218) will increase coverage by 0.3%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #1444     +/-   ##
=======================================
+ Coverage   77.3%   77.7%   +0.3%     
=======================================
  Files        128     130      +2     
  Lines       6796    6924    +128     
=======================================
+ Hits        5256    5382    +126     
- Misses      1290    1292      +2     
  Partials     250     250             

Impacted Files	Coverage Δ
bridge/opencensus/aggregation.go	100.0% <100.0%> (ø)
bridge/opencensus/exporter.go	100.0% <100.0%> (ø)
exporters/otlp/otlpgrpc/connection.go	86.9% <0.0%> (-1.9%)	⬇️

[dashpole]

Friendly ping for reviews!

[dashpole]

rebased again. @jmacd I think you said you wanted to take a look at this.