fix(testing): emit markers for benchmarks using b.Loop()

[not-matthias]

Rough outline of what internally happens and why we didn't emit markers:

Working case (RunParallel/Run/b.N):
runN: StartTimer() → b.startTimestamp = X
User: StopTimer() → (timer off, but we don't care about this pair)
User: StartTimer() → b.startTimestamp = Y
RunParallel executes
User: StopTimer() → endTimestamp = Z, appends (Y, Z) to arrays ✓
runN: StopTimer() → timer already off, does nothing

Broken case (b.Loop()):
runN: StartTimer() → b.startTimestamp = X
Loop: StopTimerWithoutMarker() → timer off, NO timestamp captured ✗
Loop body executes
Loop: StartTimerWithoutMarker() → timer on, NO new startTimestamp ✗
runN: StopTimer() → timerOn is false (from Loop), does nothing ✗
Result: No timestamps ever added to arrays!

[Copilot]

Pull request overview

This PR fixes a bug where benchmarks using b.Loop() were not emitting performance markers, preventing proper performance profiling. The issue occurred because b.Loop() uses StopTimerWithoutMarker() internally, leaving the timer off when runN() attempts to call StopTimer(), which then skips marker emission.

Key Changes

• Extracted marker emission logic into a new AddBenchmarkMarkers() function to enable reuse
• Added explicit marker emission in loopSlowPath() when the benchmark loop completes
• Updated the patch file to include these changes in the fork's build process

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
testing/testing/codspeed.go	Added new AddBenchmarkMarkers() helper function to encapsulate the logic for adding benchmark start/stop timestamps with validation
testing/testing/benchmark.go	Refactored StopTimer() to use AddBenchmarkMarkers() and added explicit marker emission in loopSlowPath() to fix missing markers for b.Loop() benchmarks
testing/patches/benchmark_benchmarkers_bloop.patch	Added patch file containing the benchmark.go changes to be applied during the fork build process
testing/fork.sh	Updated fork script to apply the new patch file

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[codspeed]

CodSpeed Performance Report

Merging #42 will improve performances by ×2.3

_{Comparing cod-1760-investigate-missing-flamegraph-for-opentelemetry (3ba284c) with main (a6c2549)}

Summary

⚡ 2 improvements
✅ 46 untouched

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	bench_collect_results[10000000, 5]	2.8 s	2 s	+39.9%
⚡	bench_go_runner	59.6 s	26.4 s	×2.3