How to know a parent is remote when starting a span

[tsloughter]

Whether or not the parent of a span that is being started is remote is an attribute that must be passed to samplers.

Span creation docs do mention remote parents but it isn't clear how it is deciding it is remote:

tracer.StartSpan(options.WithNoParent(), options.WithParentContext(parentCtx)) will generate a new child span with remote parent parentCtx, and WithNoParent will be ignored.

Why is it assumed that parentCtx is remote here?

In OpenCensus we had a optional remote parent flag that could be set when calling start_span.

[tsloughter]

Based on the Java implementation it does appear the decision is based on if the parent is a Span or SpanContext. This doesn't work well for at least the Erlang implementation where the user will only ever have the context for use as a parent.

I think there should be a explicit is parent or similar option.

[mwear]

Typically, a SpanContext that has crossed a process boundary will have been extracted with a propagator. This could be solved by adding a boolean property to indicate that on the SpanContext. Possible names could be Is/WasRemote, Is/WasExtracted. Since SpanContext is a sealed class I assume this would require buy-in from the community and a change to the spec.

[tsloughter]

@mwear yea, that is what OpenCensus did. For now I'm adding a is_remote to span context to move forward.

How to propose adding this officially to the SpanContext? Just a PR to update the spec or does it belong in an rfc first?