fix(grpc): aio grpc client interceptor incorrect metadata handling

[legau]

Description

This fixes a bug when using grpcio>=1.60.0 and the gRPC telemetry. Introduced by grpc/grpc#34347 a tuple metadata is always converted to a Metadata object which follows the Python grpc docs and typing. In the current opentelemetry implementation there is a conversion to an OrderedDict which causes :

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = Metadata((('key', 'value'),)), key = ('key', 'value')

    def __getitem__(self, key: MetadataKey) -> MetadataValue:
        """When calling <metadata>[<key>], the first element of all those
        mapped for <key> is returned.
        """
        try:
>           return self._metadata[key][0]
E           KeyError: ('key', 'value')

This PR makes use of the Metadata object directly and propagates it as such.
This could cause regressions for some following interceptors handling metadata as tuples (same way this regression happened), I'm not sure if this would qualify as a breaking change though.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Added metadata in grpc tests

Does This PR Require a Core Repo Change?

• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

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

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: legau / name: Léni (3c1c5a9, d3e3a1e)
• ✅ login: lzchen / name: Leighton Chen (038f9e5)
• ✅ login: xrmx / name: Riccardo Magliocchetti (923a569, a294d7f)

[aabmass]

Thanks for the PR @legau

This fixes a bug when using grpcio>=1.60.0 and the gRPC telemetry. Introduced by grpc/grpc#34347

Do you know if this change is backward compatible to older gRPC versions before v1.60.0?

[legau]

This transforms the tuple of <1.60 into a Metadata object so yes it is backwards compatible

[legau]

I added the changelog entry