impl(v3): add generator support for bespoke WaitForConsistency method

[scotthart]

In order to maintain feature parity from the deprecated handwritten class and avoid re-introducing threading issues previously resolved (#7740), the generator has been updated to emit code for a method not defined in the proto file. This support is intended as a one-off and is very specific to the bigtable WaitForConsistency method.

The ConnectionImpl::WaitForConsistency implementation must be handwritten in a separate .cc file.

[gemini-code-assist]

Summary of Changes

Hello @scotthart, 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 introduces a mechanism to support "bespoke methods" within the code generation framework, primarily to address the WaitForConsistency method in the Bigtable client library. By allowing the generator to declare these methods in client and connection interfaces, it facilitates the migration from handwritten classes to generated ones while preserving critical, non-proto-defined functionality and avoiding re-introduction of past threading issues. The implementation of these bespoke methods in the ConnectionImpl class is intentionally left for manual coding.

Highlights

• New Bespoke Method Configuration: Introduced a new BespokeMethod message and a repeated bespoke_methods field in generator/generator_config.proto to define methods not present in proto files but required for generated clients.
• WaitForConsistency Support: Specifically implemented support for the WaitForConsistency method, which was previously handwritten for Bigtable, to be declared in generated client and connection interfaces.
• Code Generation Logic Update: Modified the code generator to parse and emit declarations for these bespoke methods in client, connection, and connection implementation files.
• Hardcoded Documentation: Added specific Doxygen comments for the WaitForConsistency method directly within the generator (client_generator.cc) to ensure proper documentation for the generated code.
• Integration Test Updates: Updated golden integration test files to include the generated WaitForConsistency method, ensuring the new generation logic works as expected.
• Handwritten Implementation Requirement: Explicitly noted that the implementation for bespoke methods in ConnectionImpl is not generated and must be handwritten, maintaining flexibility for complex logic.

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[gemini-code-assist]

Code Review

This pull request adds generator support for a bespoke WaitForConsistency method, primarily for feature parity with Bigtable's handwritten client. The changes introduce a BespokeMethod configuration in the generator, update various generator components to handle this new configuration, and add corresponding tests. The implementation is a good starting point, but there are a few areas for improvement regarding code formatting, consistency in test documentation, and the robustness of string manipulations, which could enhance maintainability.

[codecov]

Codecov Report

❌ Patch coverage is 21.31148% with 48 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.86%. Comparing base (9429a07) to head (4407adf).
⚠️ Report is 2 commits behind head on prepare-for-v3.0.0.

Files with missing lines	Patch %	Lines
generator/internal/client_generator.cc	12.50%	14 Missing ⚠️
generator/internal/connection_generator.cc	12.50%	14 Missing ⚠️
generator/internal/service_code_generator.cc	16.66%	10 Missing ⚠️
...gration_tests/golden/v1/golden_rest_only_client.cc	0.00%	4 Missing ⚠️
generator/internal/connection_impl_generator.cc	20.00%	4 Missing ⚠️
...ion_tests/golden/v1/golden_rest_only_connection.cc	0.00%	2 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                   @@
##           prepare-for-v3.0.0   #15914      +/-   ##
======================================================
- Coverage               92.87%   92.86%   -0.02%     
======================================================
  Files                    2343     2343              
  Lines                  216117   216178      +61     
======================================================
+ Hits                   200719   200743      +24     
- Misses                  15398    15435      +37     

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