Sampling API needs to be revisited

[lmolkova]

https://github.com/open-telemetry/oteps/blob/master/text/0006-sampling.md

[lmolkova]

this was accidentally closed via PR. There are more changes coming through OTEP, so let's keep it open.

Also, spec now asks to have extra config for probability sampler

The default behavior is to apply the sampling probability only for Spans that are root spans (no parent) and Spans with remote parent. However there should be configuration to change this to "root spans only", or "all spans".

[CodeBlanch]

@cijothomas @rajkumar-rangaraj FYI I'm working on bringing us up to spec as it is right now with regard to Probability & ParentOrElse sampler. Should have a PR up soon, just working on tests.

Probability

• The ProbabilitySampler MUST ignore the parent SampledFlag.
  To respect the parent SampledFlag, the ProbabilitySampler should be used as a delegate of the ParentOrElse sampler specified below.
• Description MUST be ProbabilitySampler{0.000100}.

TODO: Add details about how the ProbabilitySampler is implemented as a function
of the TraceID.

ParentOrElse

• This is a composite sampler. ParentOrElse(delegateSampler) either respects the parent span's sampling decision or delegates to delegateSampler for root spans.
• If parent exists:
  • If parent's SampledFlag is set to true returns RECORD_AND_SAMPLED
  • If parent's SampledFlag is set to false returns NOT_RECORD
• If no parent (root span) exists returns the result of the delegateSampler.
• Description MUST be ParentOrElse{delegateSampler.getDescription()}.

Parent	ParentOrElse(delegateSampler)
Exists and SampledFlag is true	RECORD_AND_SAMPLED
Exists and SampledFlag is false	NOT_RECORD
No parent(root spans)	Result of delegateSampler()

[cijothomas]

Closing in favor of: #941