feat(spanner): add option for auto-tagging transactions

[sakthivelmanii]

Adds an option that automatically adds transaction tags to all read/write transactions that do not already include an explicit transaction tag. For read-only transactions, this option automatically adds a request tag to each statement that is executed in the read-only transaction. The tag name is equal to the class name + method name of the method that started the transaction. Tags are limited to max 50 characters. If the combination of class name + method name exceeds 50 characters, then the last 50 characters are used as the tag.

This option can be enabled with SpannerOptions$Builder.enableAutoTagging() or by setting either the environment variable SPANNER_ENABLE_AUTO_TAGGING=true or the system property spanner.enable_auto_tagging=true.

[gemini-code-assist]

Code Review

This pull request introduces an automatic tagging feature for Spanner transactions and requests, allowing the library to derive tags from the application's stack trace when they are not explicitly provided. Key changes include the addition of AutoTagHelper for stack trace analysis and updates to AbstractReadContext, SessionImpl, and SpannerOptions to support and configure this mechanism. Review feedback focuses on performance optimizations, specifically suggesting that the auto-tag be cached within the request context to avoid redundant stack trace captures and recommending the use of StackWalker for more efficient frame retrieval. Additionally, a recommendation was made to defensively copy the package list in SpannerOptions to maintain immutability.

[olavloite]

Can we update both the pull request title and add a new paragraph at the start of the pull request description? The current title and description are very technical and 'internal', instead of focusing on what's in it for a customer.

Suggestion:

feat(spanner): add option for auto-tagging transactions

Adds an option that automatically adds transaction tags to all read/write transactions that do not already include an explicit transaction tag. For read-only transactions, this option automatically adds a request tag to each statement that is executed in the read-only transaction. The tag name is equal to the class name + method name of the method that started the transaction. Tags are limited to max 50 characters. If the combination of class name + method name exceeds 50 characters, then the last 50 characters are used as the tag.

This option can be enabled with `SpannerOptions$Builder.enableAutoTagging()` or by setting either the environment variable SPANNER_ENABLE_AUTO_TAGGING=true or the system property spanner.enable_auto_tagging=true.

[olavloite]

LGTM, but please also take a look at #13214 (comment)