oc_trace:start_span/2,3 accepts undefined SpanCtx

[essen]

It calls the internal newspan function which accepts undefined.

This is to make Dialyzer pass when calling the function directly to create a new trace that is independent from the one managed by ocp.

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[tsloughter]

Thanks! I think we should also consider making the span_ctx type it self #span_ctx{} | undefined because undefined is used to act as a noop trace and it would probably be best to force that all functions taking a spanctx support the noop trace. But this is good for now.

[essen]

I wasn't sure about that but that's good to know. I can probably do a better patch next week if you want.

[deadtrickster]

@essen if this not blocking, could wait I guess.

[essen]

It's just Dialyzer complaining. Me I can wait, up to you if you want to merge earlier.

[tsloughter]

I was just going to merge this, don't know that switching span_ctx itself to also be possibly undefined will be a quick change so may as well get this in.

But @essen has to accept the CLA first :).

[essen]

I signed it!

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