Add sampling API

[reyang]

Add sampling API which follows https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/api-sampling.md.

[pyohannes]

Some notes:

• This is the related spec.
• When working on designing the interface, consider the project's ABI policy.
• This work depends on the SpanContext, which is not available yet. Coming up with a SpanContext implementation is probably a prerequisite.
• Ideally, first open a PR with the proposed sampler API, then work on the implementation of individual samplers once this is approved.
• Consider implementations in other projects:
  • Java
  • Go
  • Python

[reyang]

More notes:

• Algorithm to ensure consistent probability sampling across services - draft spec.

[reyang]

Related PRs: #118 #122 #125 #126 #128 #136 #142 #119 #136 #175 #199 #238