Have a flag to report failed Extraction

[carlosalberto]

Consider adding a second return value for Extract(), that reports whether there was an invalid payload in the carrier or it simply was empty.

From #671 :

it is impossible to distinguish "error" from "no context at all". I could imagine one could use that info to e.g. stop trying more propagators if a propagator found its headers but they were invalid. I think there should be some way for propagators to indicate this. E.g. add a second return value (this is the spec, so it's up to languages how to implement this second return value).

[Oberon00]

Quoting myself from #671 (comment):

I would suggest updating Extract() as follows:

Return values:

• [as before:] A new Context derived from the Context passed as argument,
  containing the extracted value, which can be a SpanContext,
  CorrelationContext or another cross-cutting concern context.
• [new:] A status indicator object. It must be possible to programatically detect at least: "context successfully extracted" and "no context found" and to distinguish them from any other errors the propagator encountered (e.g. header was present but value was invalid).

Some languages might implement this with an out parameter, or a class containing both values, whatever is more idiomatic.