refactor: simplify metadata reading in grpc_handler.py

[ishymko]

Metadata is a Collection so iterating over key-value tuples is possible, there are existing tests in test_grpc_handler.py.

Re #676

[gemini-code-assist]

Summary of Changes

Hello @ishymko, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refactors the metadata handling within the grpc_handler.py file. By recognizing that gRPC's Metadata object behaves as a Collection, the code for extracting metadata values has been significantly simplified, removing redundant type checks and imports. This change improves code readability and maintainability without altering functionality.

Highlights

• Metadata Handling Simplification: Refactored the _get_metadata_value function to simplify how gRPC invocation metadata is read, leveraging its Collection nature.
• Type Hinting and Import Cleanup: Removed unnecessary TYPE_CHECKING block and imports for MetadataType and Metadata as they are no longer explicitly needed for type checking or runtime logic.

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[gemini-code-assist]

Code Review

This pull request simplifies metadata reading in grpc_handler.py by removing special handling for grpc.aio.Metadata and treating it as a generic collection. This is a good refactoring that makes the code cleaner and removes unnecessary imports. I've added one minor suggestion to further condense the logic into a single list comprehension for improved conciseness and efficiency.