Bridging to opentelemetry-go

[asuffield]

The original intention was to have full compatibility from OpenCensus to OpenTelemetry. There is now a bridge to connect OpenTracing into OpenTelemetry, but because OpenCensus was not created with an API/SDK separation, some changes will be needed on this side to make it compatible. This is notably important when using a third-party library that currently has OpenCensus instrumentation.

I'm not fully clear on the details of what will be necessary here. Notable target features:

• context.Context objects can be passed between libraries using OpenCensus and libraries using OpenTelemetry without losing span parents
• if an application configures an OpenTelemetry exporter, a library using OpenCensus should send data to it