Propagate 8 bits of W3C trace context flags

[jmacd]

Description

The W3C trace context propagator currently masks unknown trace flags bits, here:

opentelemetry-go/propagation/trace_context.go

Line 50 in da2949b

flags := sc.TraceFlags() & trace.FlagsSampled

Expected behavior

Nothing in the specification explicitly says what the SDKs are meant to do here, but I believe the expectation for W3C trace context propagation is that all flags are propagated, even the unrecognized ones. The W3C specification says there are 8 flags, and I think they should all propagate.

When SDKs do not propagate unknown flags, it makes it difficult for new flags to be deployed in a timely manner. In particular, the Sampling SIG is advising OpenTelemetry not to strictly enforce the new W3C Trace Context Level 2 Random flag until this fix is widely deployed.

See open-telemetry/opentelemetry-specification#3924, https://github.com/open-telemetry/oteps/blob/main/text/trace/0235-sampling-threshold-in-trace-state.md, open-telemetry/opentelemetry-specification#3910 where I am drafting proposed sampler changes, hence I discovered this issue. I have a draft with changes that would address the proposed-clarified specification here: main...jmacd:opentelemetry-go:jmacd/randomness but I want to make sure the spec change is generally agreed to before submitting.

[MrAlias]

Won't this violate this:

... [t]he vendor will only parse the trace-flags values supported by this version of this specification and ignore all other values. [...] Vendors will set all unparsed / unknown trace-flags to 0 on outgoing requests.

[jmacd]

Yes, my apologies. For what it's worth, there is a thread on this here.