feat(baggage): fix DatadogPropagator::fields

[iunanua]

What does this PR do?

• change DatadogCompositePropagator to obtain keys from injectors according to Otel spec.
• remove the not needed DatadogPropagator::fields property

[paullegranddc]

Are you sure this is what's described in the spec?
Because from what I see, thje Keys section is under the TextMap Extract title.

The doc specific to B3 headers mentions

Fields MUST return the header names that correspond to the configured format, i.e., the headers used for the inject operation

but it seems very specific to B3 🤔

[bantonsson]

This is soo confusing... is it keys or fields? They are not always the same.

[iunanua]

Indeed.
I'm going to rename the keys() as fields()

[iunanua]

Are you sure this is what's described in the spec? Because from what I see, thje Keys section is under the TextMap Extract title.

The doc specific to B3 headers mentions

Fields MUST return the header names that correspond to the configured format, i.e., the headers used for the inject operation

but it seems very specific to B3 🤔

There is some confusion about this:
Otel TextMapPropagator trait defines the fields() method. It returns an iter of fields used by [TextMapPropagator]
and spec says

Fields MUST return the header names that correspond to the configured format, i.e., the headers used for the inject operation.

But our DatadogCompositePropagator has the keys() method. It returned the header keys of the configured extractors which were ultimately used as values for the otel fields...

[bantonsson]

@paullegranddc With regards to what fields should return, the relevant part of the generic spec is here and it says:

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