Adding FirstCookieRequest metric.

This metric tracks the type (Set or Get) of the very first call made to
cookies during a user's session.

Change-Id: Ice9e997a1680d2f842202b1cd4588cfa98f4b864
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5646502
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Olivier Li <olivierli@chromium.org>
Commit-Queue: Alex Attar <aattar@google.com>
Cr-Commit-Position: refs/heads/main@{#1323024}