fix: ensure gRPC channel options preserved during channel refresh

[axyjo]

The BigtableDataClient periodically refreshes its gRPC channel. However, when create_channel() is called during the refresh process, it doesn't include the unlimited message size options that are set during initial channel creation in init. This causes responses larger than 4 MB to fail with a ResourceExhausted error post-channel-refresh.

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Fixes #1240 🦕

[gemini-code-assist]

Summary of Changes

Hello @axyjo, 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 resolves an issue in the BigtableDataClient where gRPC channel configuration, particularly for handling large message sizes, was not correctly reapplied during channel refresh operations. The fix ensures that the client consistently uses the necessary options to prevent ResourceExhausted errors when processing large responses, thereby improving the robustness and reliability of data retrieval from Bigtable.

Highlights

• Bug Fix: Channel Options Lost During Refresh: Addressed a critical bug where gRPC channel options, specifically those allowing for unlimited message sizes, were not preserved when the BigtableDataClient refreshed its gRPC channel. This led to ResourceExhausted errors for responses larger than 4 MB after a channel refresh.
• Centralized gRPC Options Definition: Introduced a global constant GRPC_CLIENT_OPTIONS to define the default gRPC options for unlimited message send and receive lengths. This centralizes the configuration and ensures consistency.
• Consistent Channel Creation: Modified both synchronous and asynchronous create_channel methods to automatically apply GRPC_CLIENT_OPTIONS if no custom options are explicitly provided. This guarantees that refreshed or newly created channels always respect the large message size limits.
• Refactored __init__ Methods: Updated the __init__ methods in both grpc.py and grpc_asyncio.py to remove the hardcoded options list, relying instead on the create_channel logic to apply the GRPC_CLIENT_OPTIONS.

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[gemini-code-assist]

Code Review

This pull request refactors the handling of gRPC client options for message size limits in both synchronous and asynchronous Bigtable gRPC transports. Specifically, the grpc.max_send_message_length and grpc.max_receive_message_length options, previously hardcoded to -1 within the __init__ methods of the transport classes, are now consolidated into a new global constant GRPC_CLIENT_OPTIONS. This constant is then consistently applied during channel creation via the create_channel methods in both grpc.py and grpc_asyncio.py modules, ensuring that large rows can be sent and received. There were no specific review comments provided for these changes.

[axyjo]

(will close for now -- as per the parent issue, it might've been fixed in the next minor release)