llo/datasource: consistent cache expiry calculation, reduce lock contention

[calvwang9]

follow up to #21189

• AddMany: ensure consistent expiry time calculation across an atomic batch add
• Get and GetMany: reduce lock contention by moving prom metric updates outside of the read lock

[github-actions]

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[trunk-io]

     

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[cl-sonarqube-production]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[Copilot]

Pull request overview

This PR is a follow-up to PR #21189 that improves the LLO datasource observation cache implementation. It focuses on two specific optimizations: ensuring consistent expiry time calculation across batch operations and reducing lock contention during cache reads.

Changes:

• Modified AddMany to calculate expiry time once before the loop, ensuring all items in a batch have identical expiry timestamps
• Refactored Get and GetMany methods to defer Prometheus metric updates until after releasing read locks, reducing lock contention
• Added comprehensive concurrency tests for AddMany and combined AddMany/GetMany operations

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
core/services/llo/observation/cache.go	Refactored Add/AddMany to calculate expiry before locking; moved metric updates outside read locks in Get/GetMany
core/services/llo/observation/cache_test.go	Added TestCache_ConcurrentAddMany and TestCache_ConcurrentAddManyGetMany to verify thread safety

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