feat(opencensus-shim): implement OpenCensus metric producer

[aabmass]

Which problem is this PR solving?

Part of #3461

Short description of the changes

Implements OpenCensus Metric Producer, which acts as the metrics bridge. See attached bug.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update coming in follow up PR docs: update OpenCensus shim example for metrics #4078

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Added several new tests
• Will update example and docs in next PR

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated coming in follow up PR docs: update OpenCensus shim example for metrics #4078

[codecov]

Codecov Report

Merging #4066 (56f2ecf) into main (937cfaa) will increase coverage by 0.06%.
The diff coverage is 98.87%.

❗ Current head 56f2ecf differs from pull request most recent head f931a38. Consider uploading reports for the commit f931a38 to get more accurate results

@@            Coverage Diff             @@
##             main    #4066      +/-   ##
==========================================
+ Coverage   92.24%   92.30%   +0.06%     
==========================================
  Files         326      328       +2     
  Lines        9299     9385      +86     
  Branches     1971     1993      +22     
==========================================
+ Hits         8578     8663      +85     
- Misses        721      722       +1     

Files Changed	Coverage
...al/packages/shim-opencensus/src/trace-transform.ts	ø
...l/packages/shim-opencensus/src/metric-transform.ts	98.38%
...es/shim-opencensus/src/OpenCensusMetricProducer.ts	100.00%
...erimental/packages/shim-opencensus/src/ShimSpan.ts	100.00%
...imental/packages/shim-opencensus/src/ShimTracer.ts	100.00%
...mental/packages/shim-opencensus/src/propagation.ts	100.00%

[legendecas]

LGTM and it would be better to add a code document about the difference mentioned above.

[pichlermarc]

Looks good; I think we need to add one copyright notice then this should be good to merge. 🙂

[aabmass]

@pichlermarc done

[aabmass]

Good to merge?