feat(storage): expose metadata parameter in asyncio gRPC calls

[ankitaluthra1]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes#17633 🦕

[gemini-code-assist]

Warning

Gemini encountered an error creating the review. You can try again by commenting /gemini review.

[ankitaluthra1]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces metadata tracking and propagation across several asynchronous components in the Google Cloud Storage library, specifically updating AsyncAppendableObjectWriter, AsyncMultiRangeDownloader, and AsyncGRPCClient to support and persist custom metadata, timeouts, and retries. Corresponding unit tests have been added to verify these behaviors. The review feedback highlights a critical issue in both append() and download_ranges() where custom metadata passed directly to these methods is not persisted to the instance's self.metadata attribute, which would cause stream resumption and retry logic to incorrectly fall back to the older metadata.

[ankitaluthra1]

Waiting on #17616

[parthea]

I'm going to add do not merge based on #17634 (comment) (this depends on #17616)