django: Fix carrier usage on ASGI requests

[adamantike]

Description

For ASGI requests, we must use request.scope instead of request.META. This is because ASGIGetter retrieves the headers key from the carrier [0], which is only present in request.scope.

[0] https://github.com/open-telemetry/opentelemetry-python-contrib/blob/main/instrumentation/opentelemetry-instrumentation-asgi/src/opentelemetry/instrumentation/asgi/__init__.py#L133

Type of change

• Bug fix (non-breaking change which fixes an issue)
• This change requires a documentation update

How Has This Been Tested?

• Tested Django ASGI application along with the aws propagator and requests including header: X-Amzn-Trace-Id: Root=1-5759e988-bd862e3fe1be46a994272793; Parent=53995c3f42cd8ad8; Sampled=1

Does This PR Require a Core Repo Change?

• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[owais]

would be nice to add a small test case so we detect breakage with ASGI context extraction.

[adamantike]

Happy to provide a test, but didn't find an easy way to do it, without adding a propagator as dependency, or mocking attach/extract and validating what is received there.

If you have a better test scenario in mind, I can definitely implement it and add it to this PR.

[adamantike]

@owais, I was able to add a test by taking advantage of how the default propagator works, so we check the provided traceparent is retrieved from the carrier both for WSGI and ASGI.

[owais]

propagator as test dep sounds fine. I think all instrumentations that propagate context should do this if they don't already.