feat(bigtable): Add DirectPath support via env var

[hanshu-G]

Add features for supporting Directpath. This includes

• Expose an env var called CBT_ENABLE_DIRECTPATH and remove the GOOGLE_CLOUD_ENABLE_DIRECT_PATH in client to turn on Directpath
• If CBT_ENABLE_DIRECTPATH is set, use c2p::// prefix for DataEndpoint.
• Remove GrpcNumChannelsOption set being to 1
• If env var is set, attach traffic_director_enabled and direct_access_requested in the feature flags.

[gemini-code-assist]

Summary of Changes

Hello @hanshu-G, 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 client-side configuration for enabling and utilizing Directpath with Bigtable. The changes streamline the activation process through a dedicated environment variable, simplify the data endpoint URL, and remove redundant gRPC channel settings, aligning the client's behavior more closely with gRPC's native Directpath capabilities. These updates aim to improve clarity and efficiency in how Directpath is configured and communicated to the Bigtable service.

Highlights

• Directpath Activation: The mechanism for enabling Directpath has been updated. It now uses a new environment variable, CBT_ENABLE_DIRECTPATH, which accepts 'true' or '1' to activate Directpath, replacing the previous GOOGLE_CLOUD_ENABLE_DIRECT_PATH variable and its comma-separated value parsing.
• Data Endpoint Configuration: When Directpath is enabled, the DataEndpointOption is now set to c2p://bigtable.googleapis.com, simplifying the endpoint specification compared to the previous google-c2p:///directpath-bigtable.googleapis.com.
• gRPC Channel Management: The explicit setting of GrpcNumChannelsOption to 1 when Directpath is active has been removed. This change acknowledges that gRPC's internal load balancing handles channel management effectively with Directpath, making the client-side override unnecessary.
• Feature Flag Updates: The FeaturesMetadata() function now conditionally sets traffic_director_enabled and direct_access_requested to true in the FeatureFlags proto when CBT_ENABLE_DIRECTPATH is active, ensuring proper signaling to the Bigtable service.

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[gemini-code-assist]

Code Review

This pull request adds support for DirectPath in Bigtable by introducing a new environment variable and updating feature flags and connection options. The changes are generally good, but I've found a few issues: significant code duplication that violates the repository's style guide, a stale comment that could cause confusion, and a test assertion that is now incorrect due to the changes. I've provided suggestions to address these points.

[sushanb]

/gcbrun

[hanshu-G]

/gcbrun

[diegomarquezp]

/gcbrun

[codecov]

Codecov Report

❌ Patch coverage is 93.54839% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.96%. Comparing base (113d8fa) to head (0044ce1).

Files with missing lines	Patch %	Lines
...e/cloud/bigtable/internal/bigtable_stub_factory.cc	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15884      +/-   ##
==========================================
- Coverage   92.96%   92.96%   -0.01%     
==========================================
  Files        2460     2460              
  Lines      228595   228598       +3     
==========================================
+ Hits       212504   212505       +1     
- Misses      16091    16093       +2     

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