Clarify the requirements of short `trace-id` in section 8.4

[justinfoote]

Several of our engineers have been tripped up by the language in section 8.4 -- they've interpreted it to mean that they should accept a traceparent with a short traceId, like this:

00-24160746bbaecabc-393951cc1a8d5e9e-00

There is a test asserting that implementations restart the trace if such a traceparent header is received: https://github.com/w3c/trace-context/blob/master/test/test.py#L303

I'd like us to clarify that the non-normative section 8.4 only applies to accepting short traceIds from legacy non-compliant tracers in a non-w3c payload format, and that it doesn't change the normative traceparent requirements in section 3.2.

[dyladan]

So this is for accepting, for example, the jaeger request format which has a shorter trace ID, and propagating it using trace context?

[justinfoote]

@dyladan that is my understanding, yes.

[dyladan]

Actually it looks to me like this is the opposite based on current wording. It is what a system that cannot process a 32 character trace id should do when it receives a full trace id. Maybe guidelines should be added for the opposite situation like the one I described also.

[justinfoote]

Interesting. I hadn't read it that way, but I can see how this section might have been intended for that.
This text:

For better interoperability between a fully compliant systems with these existing systems, the following practices are recommended:

Doesn't make things totally clear though. These practices are recommended to whom? To a compliant system trying to interop, or to a non-compliant system trying to appear as compliant as possible?

[dyladan]

I mean you can't make recommendations to a non-compliant system. If that system can change it should just become compliant.

[SergeyKanzhelev]

@justinfoote it was never the intent to accept shorter ID. The section is suggesting that if you can only operate with 16 bytes - you need to prepend it with zeroes.

[justinfoote]

OK. I'm feeling like my changes haven't actually increased clarity, and perhaps the existing text is clear enough. I'll close this issue and the related PR.