Metrics: Renew Blink AsyncScriptCount metric

We have no intention of removing the metric, so this CL renews it before it expires.

Bug: 1242474
Change-Id: I68ba62073398d371448cafb5dab32b90ee99a2dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3141537


Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Commit-Queue: Dominic Farolino <dom@chromium.org>
Cr-Commit-Position: refs/heads/main@{#919286}