Remove effect constraint from TextMapPropagator

[iRevive]

Motivation

While working on #325, it turned out that the effectful injection into the mutable carrier was optional.
The majority of the typelevel ecosystem revolves around immutable data structures.

So, is there any actual use case for the effectful injection into the mutable carrier?

The changes

• Move propagation-specific functionality to the context.propagation package
• TextMapPropagator - add def fields: List[String]
• TextMapPropagator - remove def inject[A: TextMapUpdater](...): F[Unit]
• TextMapPropagator - rename def injected[A: TextMapUpdater](...): A to inject
• TextMapPropagator and ContextPropagators - remove effect F constraint
• Add human-friendly toString
• Add tests

What does the spec say?

https://opentelemetry.io/docs/specs/otel/context/api-propagators/#inject

Note

Injects the value into a carrier. For example, into the headers of an HTTP request.

Required arguments:

• A Context. The Propagator MUST retrieve the appropriate value from the Context first, such as SpanContext, Baggage or another cross-cutting concern context.
• The carrier that holds the propagation fields. For example, an outgoing message or HTTP request.

The new implementation complies with this requirement.

---

https://opentelemetry.io/docs/specs/otel/context/api-propagators/#textmap-inject

Warning

Injects the value into a carrier. The required arguments are the same as defined by the base Inject operation.

Optional arguments:

• A Setter to set a propagation key/value pair. Propagators MAY invoke it multiple times in order to set multiple pairs. This is an additional argument that languages are free to define to help inject data into the carrier.

Since we use TextMapUpdater rather than TextMapSetter, in some sense, we violate the requirement.

[NthPortal]

left a few comments, but nothing blocking. overall LGTM!

[NthPortal]

Since we use TextMapUpdater rather than TextMapSetter, in some sense, we violate the requirement.

if we rename TextMapUpdater to TextMapImmutableSetter, does that make us compliant? 😅

[iRevive]

I ended up renaming create to of.
What if we keep apply for simple constructors and of - for the constructors with varargs where we do some 'magic' under the hood (e.g. flattening out Multi instances)?

[NthPortal]

is there anything else this needs before merging?

[iRevive]

Nope, I can merge it right away.