Support Spanner Emulator Host for SpannerChangeStreamsToPubSub Templates

[hunsche]

This CL updates SpannerConfig initialization in the SpannerChangeStreamsToPubSub template to explicitly respect the SPANNER_EMULATOR_HOST environment variable, restoring standard Google Cloud client behavior.

The SPANNER_EMULATOR_HOST variable is the official standard across all Google Cloud client libraries (Go, Java, Python, Node) for discovering the emulator. By default, the underlying Spanner client automatically reads and uses this variable to establish plaintext connections natively.

However, Apache Beam's SpannerIO inadvertently overrides this default fallback behavior. To fix this, this CL checks for the presence of SPANNER_EMULATOR_HOST. If present, it maps it directly to .withEmulatorHost() before falling back to the template's runtime options.

This aligns the Beam pipeline's local development experience perfectly with the conventions of the broader Google Cloud ecosystem, eliminating the need for workarounds.

TEST=Verified via local build

[gemini-code-assist]

Summary of Changes

Hello, 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 enhances the SpannerChangeStreamsToPubSub template by integrating native support for the SPANNER_EMULATOR_HOST environment variable. This change ensures that the Spanner client within the Beam pipeline correctly identifies and connects to a local Spanner emulator, mirroring the behavior of other Google Cloud client libraries. The update streamlines local development workflows by eliminating the need for custom workarounds when using the Spanner emulator.

Highlights

• Spanner Emulator Host Support: Added explicit support for the SPANNER_EMULATOR_HOST environment variable in the SpannerChangeStreamsToPubSub template.
• Configuration Priority: Prioritized the SPANNER_EMULATOR_HOST environment variable over the spannerHost runtime option for Spanner client configuration.
• Aligned Development Experience: Ensured the Beam pipeline's local development experience aligns with standard Google Cloud client library conventions for Spanner emulator discovery.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.21%. Comparing base (1b8904c) to head (96a2da9).
⚠️ Report is 21 commits behind head on main.

Files with missing lines	Patch %	Lines
...ort/v2/templates/SpannerChangeStreamsToPubSub.java	0.00%	6 Missing ⚠️

❌ Your patch check has failed because the patch coverage (0.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #3545      +/-   ##
============================================
- Coverage     52.25%   52.21%   -0.04%     
+ Complexity     6055     5641     -414     
============================================
  Files          1040     1040              
  Lines         62956    62990      +34     
  Branches       6893     6902       +9     
============================================
- Hits          32895    32888       -7     
- Misses        27843    27877      +34     
- Partials       2218     2225       +7     

Components	Coverage Δ
spanner-templates	72.23% <ø> (-0.03%)	⬇️
spanner-import-export	69.05% <ø> (-0.04%)	⬇️
spanner-live-forward-migration	80.47% <ø> (-0.02%)	⬇️
spanner-live-reverse-replication	77.87% <ø> (-0.06%)	⬇️
spanner-bulk-migration	89.18% <ø> (-0.01%)	⬇️
gcs-spanner-dv	85.32% <ø> (-0.03%)	⬇️

Files with missing lines	Coverage Δ
...ort/v2/templates/SpannerChangeStreamsToPubSub.java	0.00% <0.00%> (ø)

... and 9 files with indirect coverage changes

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