Extend UMA histogram RequirementsSpecFetcher.HttpResponseCode

As per https://crbug.com/1439383#c3, extend the histogram and add
a note.

Bug: 1439383
Change-Id: Iafab0b21214028a7ea57b5965cf80e61da25c833
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4547323


Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Commit-Queue: Din Nezametdinov <shaikhitdin@google.com>
Cr-Commit-Position: refs/heads/main@{#1146483}