TraceContextPropagator uses ActivityContext in input PropagationContext as override instead of fallback

[Oberon00]

Affected code:

opentelemetry-dotnet/src/OpenTelemetry.Api/Context/Propagation/TraceContextPropagator.cs

Lines 53 to 59 in aad309b

	public override PropagationContext Extract<T>(PropagationContext context, T carrier, Func<T, string, IEnumerable<string>> getter)
	{
	if (context.ActivityContext.IsValid())
	{
	// If a valid context has already been extracted, perform a noop.
	return context;
	}

This will not extract a Context from the carrier if there already is one in the input context.

This is against the base class documentation which says:

opentelemetry-dotnet/src/OpenTelemetry.Api/Context/Propagation/TextMapPropagator.cs

Lines 45 to 53 in aad309b

	/// <summary>
	/// Extracts the context from a carrier.
	/// </summary>
	/// <typeparam name="T">Type of object to extract context from. Typically HttpRequest or similar.</typeparam>
	/// <param name="context">The default context to be used if Extract fails.</param>
	/// <param name="carrier">Object to extract context from. Instance of this object will be passed to the getter.</param>
	/// <param name="getter">Function that will return string value of a key with the specified name.</param>
	/// <returns>Context from it's text representation.</returns>
	public abstract PropagationContext Extract<T>(PropagationContext context, T carrier, Func<T, string, IEnumerable<string>> getter);

The default context to be used if Extract fails.

The base class docs conform to the OTel spec (or at least my understanding of it and the Java implementation), the TraceContextPropagator implementation does not. https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/context/api-propagators.md#extract

[stevejgordon]

@Oberon00 - This appears to be by design (added in #1048) since ASP.NET and ASP.NET Core automatically extract the W3C trace context when handling incoming requests, so there's no need to parse the headers a second time, adding a small amount of overhead. It, therefore, handles cases when an ActivityContext hasn't already been parsed (i.e. when ASP.NET (Core) is not involved). This seems reasonable, and I don't think the spec prohibits this. @martinjt, if @CodeBlanch agrees, I think we can close this.