Add example test to document how a propagator can be used for inject/extract only

[MrAlias]

#1212 updated the propagation API to unify the inject and extract parts of propagators given this is assumed the default case.

It is still possible to wrap a propagator in a way that it is only used to inject or extract cross-cutting concerns. This should be captured in an example test to help users understand how to do this if they need this functionality.

[adeniyistephen]

pointers and inputs, I would love to work on it.

[adeniyistephen]

pointers and inputs, I would love to work on it.

@MrAlias Kindly provide inputs and pointers for this issue. Regards.

[MadVikingGod]

@MrAlias Could you describe a situation where this would be used?

[MrAlias]

@MrAlias Could you describe a situation where this would be used?

First thing that comes to mind is a translation for B3 based service that runs in an environment where W3C is the standard. I.e. Extract incoming requests that contain B3 headers and only inject W3C headers on outgoing requests.