Impl OpenTracing default carrier traits for SpanContextState.

[thedodd]

So, after lots of digging through this lib, and refining the original issue that I opened here, it seems that one of the main things that needs to be finished up with this lib is to:

1. finish up the impl of InjectToHttpHeader for SpanContextState.
2. finish up the impls of the other OpenTracing default carrier types for SpanContextState as well (the inject & extract for TextMap & Binary).

Also, after evaluating the code a bit, it looks like we just need to come up with a convention for how to consistently communicate the different fields of the carrier.

A few other OT libs that I looked at in other languages seem to be using a key naming convention, and baggage items are simply prefixed with something like baggage- and then when extracted the prefix is removed.

We could certainly do something like that. Seems as though there is no standard for how that is supposed to be done according to the OT spec. Using a common nomenclature for the keys & baggage prefix should do the trick though.

Thoughts?

[sile]

Sorry for the too late reply.

baggage items are simply prefixed with something like baggage- and then when extracted the prefix is removed.

Since this is a convention of jaeger, it seems reasonable to implement this behavior to rustracing_jaeger. I think, it should be implemented around here.