fix: Update invocation metadata from grpc.aio.ServicerContext to use the correct syntax

[jubonni]

Description

invocation_metadata from grpc.aio.ServicerContext was treated as a property instead of a method which prevented extracting the invocation metadata

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Fixes #<issue_number_goes_here> 🦕

[gemini-code-assist]

Summary of Changes

Hello @jubonni, 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 addresses a critical bug in the gRPC server's request handling where invocation metadata was not being correctly retrieved. By adjusting how invocation_metadata is accessed on the ServicerContext, the system can now properly extract and utilize this essential information, ensuring correct processing of gRPC requests that rely on metadata.

Highlights

• gRPC Metadata Extraction Fix: Corrected the usage of context.invocation_metadata from grpc.aio.ServicerContext by calling it as a method (invocation_metadata()) instead of accessing it as a property. This resolves a bug preventing proper metadata extraction.
• Code Simplification: Removed an unnecessary elif block in _get_metadata_value that handled Sequence types, as invocation_metadata() consistently returns a grpc.aio.Metadata object.
• Test Updates: Modified existing test cases to correctly mock the invocation_metadata method using mock_grpc_context.invocation_metadata.return_value to align with the behavioral change.

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[gemini-code-assist]

Code Review

This pull request correctly fixes a bug where invocation_metadata was accessed as a property instead of a method on grpc.aio.ServicerContext. The changes in the application code and the corresponding test mocks are accurate and address the issue effectively. I've added one minor suggestion to make the code in _get_metadata_value more concise. Overall, this is a good fix.