Add OpenCensus trace bridge/shim

[aabmass]

Description

Implements the OpenCensus tracing shim/bridge. See issue for more details. For now, users must import and call install_shim() for the shim to take effect.

Part of #3203

Caveats

• SpanKind and Links cannot be translated with the shim. OpenCensus Tracer.start_span() only accepts span name, while OTel only allows setting those properties in start_span(); there is no way to set them after the fact with a pure API bridge.
• This PR does not bridge Context APIs. This will be needed for instrumentations which set the span context from extracted propagation headers.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Added many tests for mapping logic and interaction with OpenTelemetry. There are test for the "OpenTelemetry sandwich" problem, where OpenCensus and OpenTelemetry instrumentation should interoperate.

Does This PR Require a Contrib Repo Change?

Answer the following question based on these examples of changes that would require a Contrib Repo Change:

• The OTel specification has changed which prompted this PR to update the method interfaces of opentelemetry-api/ or opentelemetry-sdk/

• The method interfaces of test/util have changed

• Scripts in scripts/ that were copied over to the Contrib repo have changed

• Configuration files that were copied over to the Contrib repo have changed (when consistency between repositories is applicable) such as in

  • pyproject.toml
  • isort.cfg
  • .flake8

• When a new .github/CODEOWNER is added

• Major changes to project information, such as in:

  • README.md
  • CONTRIBUTING.md

• Yes. - Link to PR:

• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated - done in next PR Add OpenCensus tracing shim example #3211

[aabmass]

Thanks for reviews everyone. I just realized we probably need to bridge the Context APIs as well for instrumentations which extract a Context with propagators e.g. opencensus-ext-flask.

I'll work on this in a separate PR and track it in the issue.

[jsuereth]

Mostly looking at this for OC compatibility compliance.

Tests cover everything I'd expect to see. Also to confirm, this does NOT include the bridge for propagation right?

[aabmass]

Also to confirm, this does NOT include the bridge for propagation right?

Correct. I updated #3203 to track it.

[srikanthccv]

@jeremydvoss PTAL.