ExtractHTTP should stop after extracting first valid trace context

[rghetia]

There is an issue opened on spec to clarify the behavior. There are two problems highlighted in the current implementation.

1. The Last trace context is invalid and the first trace context is valid - Even though the first context is valid it will not use it.
2. Both of them are valid - The last context will be used but that may not be counter intuitive to the user. Also it involves unnecessary processing. Extracting is not cheap.

[MrAlias]

open-telemetry/opentelemetry-specification#671 merged with the added clarification:

If a value can not be parsed from the carrier for a cross-cutting concern, the implementation MUST NOT throw an exception and MUST NOT store a new value in the Context, in order to preserve any previously existing valid value.

This clarifies 1 directly.

As for 2, it seems implied that extracting both, though not optimal, is what the specification assumes to be done.

[Aneurysm9]

It seems that the correlation context propagator may be the only implementation that needs to change. Both the B3 and W3C trace context propagators return the input context unmodified if unable to extract a valid SpanContext. Are there other Propagator implementations I'm missing?

[MrAlias]

Are there other Propagator implementations I'm missing?

No, this should be all.

Did you want to take this issue @Aneurysm9?