logger: avoid mutex contention

[AskAlexSharov]

an alternative for: #20148

Move mutex from log handler to file writer. Seems we used mutex in handler - to reduce allocs (by using shared buf in the past) - but now we don't use any shared buffer there.

TestStreamHandlerNoContention 5K goroutines: 5s -> 7ms

[Copilot]

Pull request overview

This PR aims to reduce logger mutex contention by removing the global synchronization wrapper from StreamHandler and instead adding locking only where it’s required (rotating file writes), with accompanying tests/bench-oriented checks to catch contention regressions.

Changes:

• Remove SyncHandler wrapping from StreamHandler to avoid global serialization during logging.
• Add an internal mutex to rotatingWriter to make rotation (stat/truncate/write/sync) safe under concurrency.
• Add new tests in bench_test.go to detect mutex contention and measure allocation/concurrency overhead.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 6 comments.

File	Description
common/log/v3/handler.go	Removes handler-level mutex from StreamHandler; adds mutex inside rotatingWriter.Write for safe multi-step rotation.
common/log/v3/bench_test.go	Adds contention-focused and allocation/concurrency tests around StreamHandler.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

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[awskii]

Approving this as better alternative to more complicated solution which also can silently drop logs in case of bursts. Sometimes we do burst logging, especially happens with Debug logging level. Usage allows blocking logging calls which is additional contention, i think this version would be easier to support