Implement OpenCensus compatibility shim

[fbogsany]

Note that the OpenCensus implementation for Ruby was incomplete - in particular, it lacked metrics and exporter support. Do we need to implement an OC shim? Does it need to support metrics?

[johnnyshields]

@fbogsany What's the real-world justification for this? Who is only supporting OpenCensus but not OpenTelemetry?

[fbogsany]

This is more complicated than opentracing. OpenCensus for Ruby never progressed beyond Alpha. There is a tracking issue for the compatibility shim here: open-telemetry/opentelemetry-specification#1175. It is not clear whether we need to implement one for Ruby.

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