chore: use tracing spans for metrics

[jonathanpwang]

sync with openvm-org/stark-backend#81

[codspeed-hq]

CodSpeed WallTime Performance Report

Merging #1791 will not alter performance

_{Comparing chore/timer-metric-layer (4e5919e) with feat/new-execution (bb20ff8)}

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

Summary

✅ 20 untouched benchmarks

[github-actions]

group	app.proof_time_ms	app.cycles	app.cells_used	leaf.proof_time_ms	leaf.cycles	leaf.cells_used
verify_fibair	(+74 [+5.3%]) 1,461	322,699	17,340,154	-	-	-
fibonacci	(-67 [-2.5%]) 2,618	1,500,277	50,589,503	-	-	-
regex	(+331 [+4.5%]) 7,678	4,165,432	166,519,456	-	-	-
ecrecover	(-184 [-10.3%]) 1,598	137,260	8,151,353	-	-	-
pairing	(-151 [-3.5%]) 4,178	1,862,964	97,508,739	-	-	-

Commit: 4e5919e

Benchmark Workflow