feat: refactor and deprecate databaseRole and Session Labels SessionP…

[surbhigarg92]

This PR Deprecate databaseRole and labels in SessionPoolOptions in favor of SpannerOptions.sessionLabels and the Database constructor argument. Existing configurations will continue to work with a fallback.

We plan to deprecate SessionPool and SessionPoolOptions in upcoming PR, so this PR gives an option to configure these parameters.

Changes:

• feat: add sessionLabels to SpannerOptions to allow defining session labels at the client level
• feat: update Database constructor to accept databaseRole as a direct parameter
• fix: deprecate labels and databaseRole in SessionPoolOptions

[gemini-code-assist]

Summary of Changes

Hello @surbhigarg92, 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 how session labels and database roles are configured within the Spanner client library. It introduces new, more direct ways to specify these options at the client and database levels, while gracefully deprecating the older methods previously found in SessionPoolOptions. The changes aim to streamline configuration and improve clarity, ensuring backward compatibility for existing implementations.

Highlights

• Centralized Session Label Configuration: Introduced sessionLabels in SpannerOptions to allow defining session labels at the client level, providing a more centralized configuration point.
• Direct Database Role Assignment: The Database constructor now directly accepts databaseRole, simplifying its configuration and making it more explicit.
• Deprecation of Session Pool Options: databaseRole and labels within SessionPoolOptions have been deprecated, with a clear migration path to the new SpannerOptions.sessionLabels and Database constructor argument.
• Backward Compatibility: Existing configurations using the deprecated SessionPoolOptions will continue to function with a fallback mechanism, ensuring a smooth transition.
• Updated Test Coverage: New tests have been added to validate the correct behavior of the new databaseRole argument in the Database constructor and the sessionLabels from SpannerOptions.

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[gemini-code-assist]

Code Review

This pull request effectively refactors databaseRole and labels from SessionPoolOptions, deprecating them in favor of SpannerOptions.sessionLabels and a direct Database constructor argument. The changes are well-implemented with proper fallbacks for backward compatibility, and the tests have been updated accordingly. I have one minor suggestion to improve code conciseness by using optional chaining.