OpenCensus trace bridge

[aabmass]

Implement the OpenCensus trace bridge part of the spec https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/compatibility/opencensus.md#trace-bridge. Metric bridge is tracked in #3461

Please assign this to me

[Flarna]

I have seen the new package is in folder opentelemetry-shim-opencensus. Recently we dropped the opentelemetry- prefix for new packages. Not sure if it is too late already but maybe we should rename it.

[aabmass]

Not too late as the package is still private. Thanks for letting me know, I'll send a PR to fix it.

[llc1123]

I have seen the new package is in folder opentelemetry-shim-opencensus. Recently we dropped the opentelemetry- prefix for new packages. Not sure if it is too late already but maybe we should rename it.

Is it safe removing the prefix of other packages? I don't see any potential problem as the package names keep the same.

[Flarna]

For end users it's fine. It's a bit cumbersome for devs because git is sometimes not that good in showing the history for renames.