fix: do not convert grpc.aio.Metadata to OrderedDict

[BatmanAoD]

Description

gRPC metadata has traditionally been represented as nested tuples, so when injecting new metadata, interceptors needed to convert to a mutable type and then back into a nested tuple. However, for async gRPC, the aio.Metadata class breaks this conversion, and the conversion is unnecessary anyway since the data is already mutable.

Fixes #2373

Type of change

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

How Has This Been Tested?

TODO

• Test A

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]

• ❌ - login: @BatmanAoD / name: Kyle J Strand . The commit (bbbcf27, a64b49c) is not authorized under a signed CLA. Please click here to be authorized. For further assistance with EasyCLA, please submit a support request ticket.

[BatmanAoD]

I'm not sure why I'm not covered under the CLA. When I click the "not covered" button and go through the process, it says I'm ready to go (as a corporate contributor under Rigetti), but the status of the EasyCLA check doesn't seem to change.

For comparison, in this PR, it marked me as covered: grpc/grpc-go#7038

Is it perhaps because one of my commits here is unverified? (I'm not sure why that is, but I can try to fix it if that's the problem.)

[thpierce]

I think this issue was resolved with #2363, note that the issue persisted (for me) even after grpc/grpc#26498 was completed.

[BatmanAoD]

I haven't confirmed whether or not this is resolved, but this PR isn't mergeable anyway.