Rename ProbabilitySampler to TraceIdRatioBasedSampler and add requirements

[bogdandrutu]

In #331 there were questions about why different requirements are important when deciding on the probability sampling algorithm.

[reyang]

Do we want to mention security consideration (e.g what's the trust boundary)? E.g. a malicious guy could generate trace ids that always result in the trace being sampled even at extremely low rate.

This might be solved by starting a new trace across trust boundary, there are many other solutions, too.

[bogdandrutu]

@reyang that is a good point, but I don't think it is in scope of this sampler. I think that is a more general problem and not only for having things sampled (because another way to do that is to always set the traceflag option to 1). There is a special section in w3c about security concerns.

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 7 days.

[bogdandrutu]

will work on this soon, commenting to not close it

[lizthegrey]

Thanks Bogdan & Yuri!

[bogdandrutu]

@yurishkuro @lizthegrey PR updated.

[arminru]

This is only relevant for requests that cross trust boundaries because otherwise the sampled flag in the incoming TraceState is honored by the parent-based sampler, right? This could mentioned to avoid confusion.

[bogdandrutu]

Not really, we decided that the samplers like this will not honor any flag or TraceState, is the user responsibility to configure the ParentBased:

The TraceIdRatioBased MUST ignore the parent SampledFlag. To respect the parent SampledFlag, the TraceIdRatioBased should be used as a delegate of the ParentBased sampler specified below.

[bogdandrutu]

Re-read, yes in general if parent-based is set it is not very important, but even if parent-based is not set this should be the same case.