Consider trace context support for probability sampling

The OpenTelemetry project has been working to specify how it propagates information about probability sampling through a couple of OTEP drafts:

OTEP 168: Specify how to propagate consistent head sampling probability
OTEP 170: Probability sampling: Sampler Name and Adjusted Count attributes

The first of these discusses how to propagate head probability so that each Span recorded in a Trace Context that has the sampled flag set knows its "adjusted count", which is the inverse of probability. We have proposed to use power-of-two sampling rates, following research by Otmar Ertl, and have come to see the use of a dedicated tracestate field as potentially too costly to have on-by-default.

Using tracestate means passing around 30 bytes per context, and considering this overhead we would like to see a Version-1 W3C traceparent with the addition of a couple of bytes of information. We can do this with 6 or 7 bits of information, ideally, but it will require specifying a lot more about traceparent and which bits of the TraceID are truly random.

This issue is a placeholder for raising this discussion in the W3C group.