[FedCM] Add metrics to assess how often get is called after onload (2/2)

One of our prototypes for multi IDP uses the window onload event as
a cutoff for IDP registration. If we move forward with the prototype,
single IDP requests will also be affected since RequestToken will not
be called immediately but instead after the window onload event is
fired. We want to investigate how often this delay would be introduced.

Bug: 1394438
Change-Id: I61fb38589e871ea0f758ceda2e51b05ea71c1092
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4167081
Commit-Queue: Zachary Tan <tanzachary@chromium.org>
Reviewed-by: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1094647}