Assumes that INVALID parent span id is actually null in TraceContext

[marcingrzejszczak]

• without this change we're returning an INVALID parent span even though we define in the javadocs of TraceContext that we would return null
• with this change we're verifying whether the INVALID parent is set in which case we're returning null

fixes gh-687

[jonatan-ivanov]

Since the result in the original issue was dependent on the sampling decision, should we double the tests and test sampled and not sampled scenarios as well?

[marcingrzejszczak]

You're right. The problem is that with sampler e.g. always off we can't get the parent id from OTel even though we should.

 @Test
    void should_return_parentid_when_spans_not_sampled() {
        SdkTracerProvider sdkTracerProvider = SdkTracerProvider.builder()
            .setSampler(io.opentelemetry.sdk.trace.samplers.Sampler.alwaysOff())
            .build();

        OpenTelemetrySdkBuilder openTelemetrySdkBuilder = OpenTelemetrySdk.builder().setTracerProvider(sdkTracerProvider);

        try (OpenTelemetrySdk openTelemetrySdk = openTelemetrySdkBuilder.build()) {
            Tracer otelTracer = tracer(openTelemetrySdk);
            Span parentSpan = otelTracer.spanBuilder("parent").startSpan();
            Span span = otelTracer.spanBuilder("foo")
                .setParent(parentSpan.storeInContext(Context.current()))
                .startSpan();

            OtelTraceContext otelTraceContext = new OtelTraceContext(span);

            then(otelTraceContext.parentId()).isEqualTo(parentSpan.getSpanContext().getSpanId());
        }

    }

This will not pass. I need to figure out how to solve this

[jonatan-ivanov]

we can't get the parent id from OTel even though we should

According to OTel we should never be able to get the parentId "instrumentation time", only in the processors/exporters by looking at the span tree. I think in that case it might be ok to return and assert null as we did earlier since that's the best I think we can do.

[marcingrzejszczak]

This would work differently for Brave and OTel then I guess?