Experimental: extend span APIs to support callbacks and access to spancontext

[reyang]

No description provided.

[reyang]

@bogdandrutu @c24t @discostu105 @SergeyKanzhelev

[reyang]

This is to follow up on how to allow APM vendors to integrate with OpenCensus by having more access to span/spancontext.

1. We will use Google doc for the design discussion. @discostu105 would you create an initial version?
2. A development branch experimental will be created for the pilot work in OpenCensus Python. @reyang to create the branch (let me know if you want a better branch name).
3. Once the work got validated, we will bring the design into opencensus-specs.
4. Once the design got accepted by opencensus-spec, we will merge the Python experimental branch into master, and start the implementation in other languages.
5. This effort should not affect/block the current release schedule.

[reyang]

Superseded by census-instrumentation/opencensus-specs#247.