[OpentracingShim] Support returning non-null SpanContextShim in Propagation.extractTextMap when traceId and SpanId are not present in OTel SpanContext

[ChenX1993]

Is your feature request related to a problem? Please describe.
Hi,
The request is to make Propagation.extractTextMap method able to conditionally return non-null SpanContextShim when traceId and SpanId are not present OTel SpanContext.
Currently it returns null when OTel SpanContext is invalid - no valid traceId and SpanId:

opentelemetry-java/opentracing-shim/src/main/java/io/opentelemetry/opentracingshim/Propagation.java

Lines 47 to 51 in 188210b

	Span span = Span.fromContext(context);
	Baggage baggage = Baggage.fromContext(context);
	if (!span.getSpanContext().isValid() && baggage.isEmpty()) {
	return null;
	}

The reason is to support jaeger-debug-id use case in OTel when we are migrating from deprecated jaeger-client-go.
To quickly explain what jaeger-debug-id does: it allows users to send a inbound request with such jaeger-debug-id header present to enforce sampling without a parent span.

I can implement a custom propagator to extract the header in maybe TraceState in OTel SpanContext, and implement a custom sampler to detect if it is present in the TraceState to sample the span and to set the attribute.
The only blocker right now is in OpentracingShim: it only returns a non-null bridge SpanContextShim when there OTel SpanContext is valid - traceId and SpanId are valid or baggage is not empty, so there is no way to pass the jaeger-debug-id info into the OTel when creating the span.

Describe the solution you'd like
Since the jaeger-debug-id is very specific to jaeger, and not general to OpenTracing. I am thinking about more generic way to solve it.

One approach I can think of is to make such change:

if (!span.getSpanContext().isValid() && !span.getSpanContext().isSampled && baggage.isEmpty()) {
      return null;
}

Then in my custom propagator, I can set the traceFlags to sampled when extracting jaeger-debug-id header.

or

if (!span.getSpanContext().isValid() && span.getSpanContext().getTraceState.get("force.passthrough") == null && baggage.isEmpty()) {
      return null;
}

Hope this request make sense. Thanks!

[jkwatson]

paging @carlosalberto

[ChenX1993]

If you are ok with the request, then you can assign to me.
I can contribute to that.

[carlosalberto]

I think this makes sense, as I remember seeing this in Jaeger and was curious whether we would ever need this.

@yurishkuro any opinion on this, or are we ok supporting this?

[carlosalberto]

Ping @yurishkuro

[yurishkuro]

+1