PoC: Implement MetricProducer by making the SDK a Producer

[dashpole]

Issue: #3504

Implements open-telemetry/opentelemetry-specification#2951.

This is an example of how metric.Producer could be implemented such that the SDK itself is a "special" producer. I'm also going to prototype what it would look like to keep the interfaces separate.

[codecov]

Codecov Report

Merging #3523 (b64a097) into main (4146bd1) will decrease coverage by 0.1%.
The diff coverage is 89.3%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #3523     +/-   ##
=======================================
- Coverage   78.1%   78.0%   -0.2%     
=======================================
  Files        165     164      -1     
  Lines      11809   11742     -67     
=======================================
- Hits        9228    9160     -68     
- Misses      2385    2386      +1     
  Partials     196     196             

Impacted Files	Coverage Δ
sdk/metric/manual_reader.go	80.4% <87.8%> (+1.7%)	⬆️
sdk/metric/periodic_reader.go	88.9% <90.0%> (-0.6%)	⬇️
sdk/metric/pipeline.go	95.1% <100.0%> (+1.4%)	⬆️
sdk/metric/reader.go	100.0% <100.0%> (ø)
exporters/jaeger/jaeger.go	90.3% <0.0%> (-0.9%)	⬇️
sdk/metric/view/view.go

[MrAlias]

Reviewers: looking for high-level feedback on interface design here.

[dashpole]

Closing this for now based on feedback from the sig meeting.