[grpcio] Add keys, values and items to aio.Metadata

[Blacksujit]

Since #15899 rebased grpc.aio.Metadata on Collection, metadata.keys(), metadata.values() and metadata.items() are reported as missing attributes, although the runtime class defines them.

This adds the three methods with their runtime types. Note that the views group values per key, so values() and items() yield list[str | bytes], unlike iteration over the metadata itself, which yields one (key, value) tuple per entry.

Upstream (v1.84.0): https://github.com/grpc/grpc/blob/v1.84.0/src/python/grpcio/grpc/aio/_metadata.py#L130-L137

AI agent used: Claude Code (claude-opus-5-5).

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

scipy-stubs (https://github.com/scipy/scipy-stubs)
- tests/misc/test_submodules_empty.pyi:3: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports