feat(bigtable): add expiration logic to query plan implementation

[mpeddada1]

This PR introduces expiration logic to the QueryPlan (currently returns empty string for the query and empty ResultSetMetadata). It doesn't contain a refreshing mechanism as described in https://github.com/googleapis/googleapis/blob/55319b058f8a0e46bbeeff30e374e4b1f081f494/google/bigtable/v2/bigtable.proto#L1237

[gemini-code-assist]

Summary of Changes

Hello @mpeddada1, 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 an essential expiration mechanism to the QueryPlan implementation for Bigtable. By integrating valid_until timestamps, the system can now detect and prevent the use of stale query plans, ensuring that prepared_query and metadata accessors only provide up-to-date information. This enhancement improves data integrity and sets the stage for future features like automatic refreshing, which is noted as not yet included.

Highlights

• Expiration Logic: The QueryPlan::prepared_query() and QueryPlan::metadata() methods now incorporate expiration logic, checking a valid_until timestamp.
• Stale Data Prevention: If the QueryPlan has expired (i.e., valid_until is in the past), these methods will now return empty strings or empty ResultSetMetadata objects, preventing the use of stale data.
• Thread Safety: A std::mutex (mu_) and std::lock_guard are utilized to ensure thread-safe access to the internal response_ member during expiration checks.
• Test Coverage: New unit tests have been added to query_plan_test.cc to validate both active and expired QueryPlan scenarios, ensuring the expiration logic functions as expected.

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[gemini-code-assist]

Code Review

This pull request introduces expiration logic to the QueryPlan, which is a good step forward. The implementation is straightforward and the new tests correctly verify the behavior for both valid and expired plans. I have a few suggestions to improve code quality, maintainability, and adherence to the project's style guide. My comments focus on reducing code duplication, using std::make_shared as per the Google C++ Style Guide, and improving the robustness of tests by using dedicated protobuf matchers.

[codecov]

Codecov Report

❌ Patch coverage is 90.62500% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.11%. Comparing base (04f7a48) to head (36c2a5a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
google/cloud/bigtable/internal/query_plan.cc	84.61%	2 Missing ⚠️
google/cloud/bigtable/internal/query_plan_test.cc	94.73%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15647   +/-   ##
=======================================
  Coverage   93.11%   93.11%           
=======================================
  Files        2433     2433           
  Lines      223810   223834   +24     
=======================================
+ Hits       208400   208424   +24     
  Misses      15410    15410           

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[scotthart]

Let's go ahead and move the implementation of this function into the .cc file along with the other member functions.

[mpeddada1]

Moving this implementation into the .cc file, thanks for catching this. Additionally, the implementations of the other member functions are currently present in .cc.