[Proposal] Deprecate TextMapCarrier.Keys

[pellared]

@open-telemetry/go-maintainers, do you know why TextMapCarrier has a Keys method? It does not look useful at all. I think about adding a comment that we have it just for backwards compatibility...

I have not seen Keys used anywhere besides tests.

I saw this:

If your carrier is reused, you should delete the fields here before calling Inject.

However, I do not see sense in it.

Originally posted by @pellared in #6778 (comment)

[MrAlias]

Keys is used to tell a propagator what keys to expect or are valid to send. It is used for interoperability.

This is a required field to be compliant with the specification: https://github.com/open-telemetry/opentelemetry-specification/blob/ad7f27d97e7570dc515e599d342feeddf6d8500a/specification/context/api-propagators.md#keys

We should not remove or deprecate it.

[pellared]

Thanks 🙇 I missed it when reading the first time