OpenCensus trace shim

[aabmass]

Implement the OpenCensus trace shim.

• Specification: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/compatibility/opencensus.md#trace-bridge
• Java Impl: https://github.com/open-telemetry/opentelemetry-java/tree/main/opencensus-shim

Tasks:

• Bridge the Tracer API Add OpenCensus trace bridge/shim #3210
• Bridge the Context APIs – needed for instrumentations which extract a Context with propagators e.g. opencensus-ext-flask.
• OpenCensus Binary Context Propagation
  • Implement an OpenCensus TextMapPropagator if possible
  • Add option to enable it in the gRPC instrumentation
• Revert https://github.com/open-telemetry/opentelemetry-python/pull/3227/files#diff-3d64996115d0ac3b74e36f57ce6b00d6b4982614c41b1ed39eab8ef43b2e9a2b so this package is released

[aabmass]

From #3210

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 could easily be supported if open-telemetry/opentelemetry-specification#3186 gets merged and implemented.

[aabmass]

• Bridge the Context APIs – needed for instrumentations which extract a Context with propagators e.g. opencensus-ext-flask.

I don't think bridging the OC context API To OTel is possible. OC context API has no "detach"
concept; in fact, the ContextVar.set() calls do not even save the returned token
https://github.com/census-instrumentation/opencensus-python/blob/master/context/opencensus-context/opencensus/common/runtime_context/__init__.py#L156.

Instead, the ContextTracer manually resets the previous value by calling set with the parent
span context:
https://github.com/census-instrumentation/opencensus-python/blob/2e08df591b507612b3968be8c2538dedbf8fab37/opencensus/trace/tracers/context_tracer.py#L114-L117

Meanwhile, OTel achieves this (correctly) by just using ContextVar.reset(token).

Instead, I think it will be more compatible to just continue using the OC RuntimeContext as is
and just mirror the parts we are interested in bridging into the OTel context.