OpenTelemetry instrumentation

[tsloughter]

OpenCensus was deprecated and replaced by the merging with OpenTracing resulting in https://opentelemetry.io/

We have https://github.com/open-telemetry/opentelemetry-erlang

I was originally going to simply replace the OC module in this repo but now wondering if it would be better to put it as its own library in https://github.com/open-telemetry/opentelemetry-erlang-contrib to make it more visible for someone coming to otel and looking to see what instrumentions are available.

[tsloughter]

This will be a separate library and is now open as a PR open-telemetry/opentelemetry-erlang-contrib#70