Stabilize `GetAll` on the TextMap Extract in Propagators API

[jamesmoessis]

I'm writing this issue with a lot of OTel people in the room! KubeCon London contrib fest

What are you trying to achieve?

Stabilise the GetAll function in the TextMap Extract in the spec: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/context/api-propagators.md#getall

In order to do this, we need to show that there are three prototypes across the language implementations. We already have three four prototypes / compatible languages, which I'll list below.

• Java, already implemented as experimental: Extends TextMapGetter with GetAll() method, implement usage in W3CBaggagePropagator opentelemetry-java#6852, and for instrumentation there are some instrumentations that have already implemented the extended interface: Implement ExtendedTextMapGetter in instrumentations opentelemetry-java-instrumentation#12868
• Go, a prototype exists here: propagation: extract of multiple header values opentelemetry-go#5973, which @pellared is happy to merge, with some discussed changes.
• JavaScript, already the TextMapGetter interface can return string[]: https://github.com/open-telemetry/opentelemetry-js/blob/v2.0.0/api/src/propagation/TextMapPropagator.ts#L110-L116
• Python API already supports the use case, as the Text Map Getter returns List[str]: https://github.com/open-telemetry/opentelemetry-python/blob/48fc3bfb2da53532a7a9aaa7e26afb37bc1c9276/opentelemetry-api/src/opentelemetry/propagators/textmap.py

Additional context.

#4295 added the GetAll function as "In Development"

cc @trask @pellared

[jamesmoessis]

PR is up: #4472

[carlosalberto]

Java has the incorrect PR

[pellared]

Java has the incorrect PR

Fixed