Support OpenCensus -> OpenTelemetry migration path

[nilebox]

Is your feature request related to a problem? Please describe.
The OpenTelemetry merger announcement promised straightforward backwards compatibility with OpenCensus.

We need to provide support for libraries which are currently instrumented via OpenCensus APIs to deliver their data to OpenTelemetry pipeline.

Describe the solution you'd like
#1863 Is the first "shim" version which has been merged.
It has a problem of using expiring cache which may not work well for use cases with high QPS.
We may try to implement TracerImpl which wouldn't use any "impl" code from OpenCensus at all (or use it minimally without requiring a cache).

Additional context
See discussion in #1863 for more information.

[bogdandrutu]

@nilebox can this be closed as duplicate of #2013 or the other one, and move the comments to here?