Propagation: TextMap propagator getter must provide a keys method

[codeboten]

As per specification, the Getter argument must provide a Keys method

The Keys function MUST return the list of all the keys in the carrier.

https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/context/api-propagators.md#keys

[nprajilesh]

@codeboten I'll take this up. Could you assign this to me?

[codeboten]

Done, thanks!

[nprajilesh]

Hey @codeboten , could use some help understanding this => I went through the spec and the existing codebase and my understanding is that currently when the extract method is called, a get function is passed to it as a param.

If we have to modify Getter to have both get() and keys() as functions, we will have to make Getter a class.

Doing this change would mean we would have to change how the get method is currently being passed and invoked in extractors in other files like B3format where we are just passing a method and not an object of a class

Is my understanding right? Would I have to change the files where getter is being used as well?

[codeboten]

@nprajilesh yes that is my understanding of how this feature should be implemented.

[nprajilesh]

@codeboten Could you check if the implementation in this PR #1196 is ok? If so, I will make it ready for review.