Empty headers injected by HTTP and Baggage propagator

[yzsolt]

If the HTTP propagator is used on an invalid context or the baggage propagator is used on an empty baggage, the resulting header map will contain empty values. I don't know if this is intended or not, but I think it'd be more user friendly if empty values wouldn't be injected, so the user wouldn't have to remove or check them manually before using them as HTTP request headers.

[lalitb]

A quick look into the HTTP propagator code, I don't think empty header files are injected if the context is not valid as we return early in that case:

opentelemetry-cpp/api/include/opentelemetry/trace/propagation/http_trace_context.h

Lines 40 to 43 in c8bd431

	if (!span_context.IsValid())
	{
	return;
	}

Let me know if I am missing something here.
Though this may be true for the Baggage propagator and need to be fixed if so.

[lalitb]

@yzsolt #1367 should fix the issue with baggage propagator. Can you check HTTP propagator once again, as our tests does validate that HTTP headers are not populated in case the context is invalid.

[yzsolt]

@lalitb Sorry for necro-bumping this issue, but I've finally had time to check it once again, and the Tracestate header still can be empty. I've also found the root cause: while there is a test case for this, it's checking the extracted carrier variable instead of the injected carrier2. So Tracestate header injection still needs an if and the tests should be fixed too. I'd suggest using more meaningful names for the extracted/injected carriers in all propagation tests to avoid an issue like this in the future.