The execute_us is now tied to the --slow-histograms flag.
This currently enabled on a small population as a persistent finch study
which should give us enough coverage for now.

Drive-by-fixes:
- Rename counter: execute_precise() to execute()
- Avoid Leave/Enter overhead in NestedTimedHistogramScope if the
  histogram is not enabled
- Only stop timers in debug mode for NestedTimedHistogramScope

Bug: chromium:1275056
Change-Id: Id6a492bdd68edb5194cb191c7083829a9f90283f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3320431Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78318}