Merge in OpenCensus, OpenTracing specs

[c24t]

Addresses #13.

This is a WIP PR to reconcile the opencensus and opentracing specs. The goal is to describe the java client as it stands now, without prescribing any new behaviors or language-specific implementation details.

This is WIP in that the structure and the content needs to change significantly before this is merged. I copied both specs (OT into specification/README.md and OC into specification/) to make it easy to comment on the existing specs to point out what behavior has changed. Hopefully this will help us collect these changes to describe in another doc for #12.

Open questions

Should the spec describe the API only, or should it include required SDK components as in #6?

Should the spec describe context propagation? Some options are:

• Describe the APIs for in-process context prop as implemented by gRPC, go's Context, python's contextvars, etc. here
• Describe in-process context prop at a high level, but otherwise leave it out of the spec except to note what's stored in the context
• Defer to w3c (https://github.com/w3c/trace-context, https://github.com/w3c/correlation-context) for inter-process propagation
• Include a list of required inter-process propagation formats as in Define the list of built-in propagation formats #8
• Combine in- and inter-process context prop in a separate spec as in @yurishkuro's context propagation layers proposal

Should the spec include the protos, a prose description of the data structures the protos represent, or neither?

Should Noop/Default classes be considered part of the API, SDK, or left out as an implementation detail?

Should the spec describe exporting to opencensus and opentracing? As I understand it both libraries will eventually depend on opentelemetry, but it may be helpful to client authors to describe the OC/OT shims in the meantime.

cc @bogdandrutu, @iredelmeier

[c24t]

Still TODO:

• Rename classes, remove references to opentracing and opencensus
• Import stats specs, combine stats and metrics as in Move stats to metrics opentelemetry-java#241
• Replace annotations with events as in Annotation -> Event opentelemetry-java#26
• Add top-level specs for Tagger and Meter alongside Tracer, see Add a global registry opentelemetry-java#218

[SergeyKanzhelev]

I'd suggest to create work-in-progress folder for specs the same way we did for semantics model. And copy files when complete. This way we can merge this PR as is and iteratively improve in smaller PRs.

[SergeyKanzhelev]

I'll merge as this goes to work-in-progress folder. Similar to what we did with #14. More reviewers would be needed for documents going to non-WIP folders