[PoC] Reimplement OpenCensus bridge

[dashpole]

Fixes #2808

It follows the design described in open-telemetry/opentelemetry-specification#2722, and open-telemetry/opentelemetry-specification#2732.

[codecov]

Codecov Report

Merging #3093 (70185b3) into main (d7bfe66) will decrease coverage by 0.0%.
The diff coverage is 58.1%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3093     +/-   ##
=======================================
- Coverage   77.3%   77.2%   -0.1%     
=======================================
  Files        159     160      +1     
  Lines      11139   11178     +39     
=======================================
+ Hits        8613    8638     +25     
- Misses      2329    2342     +13     
- Partials     197     198      +1     

Impacted Files	Coverage Δ
sdk/metric/config.go	91.0% <16.6%> (-9.0%)	⬇️
sdk/metric/pipeline.go	91.0% <27.7%> (-6.3%)	⬇️
bridge/opencensus/metric.go	45.1% <100.0%> (+45.1%)	⬆️
bridge/opencensus/version.go	100.0% <100.0%> (ø)
sdk/metric/provider.go	100.0% <100.0%> (ø)
exporters/jaeger/jaeger.go	91.1% <0.0%> (+0.8%)	⬆️
sdk/trace/batch_span_processor.go	81.9% <0.0%> (+0.8%)	⬆️

[dashpole]

This is now rebased on main (from the new_sdk/main branch)