bench: refactor to use string interpolation in `stats/iter` #24566

Job	Run time
Manage labels	0s
Update copyright header years	0s
Merge changes from develop branch into this PR	0s
Rebase this PR on top of develop branch	0s
Auto-fix lint errors	0s
Check for required files	0s
Remove in-progress label	0s
	0s

Provide feedback