[GWS] Add connection reuse-related metrics to PageLoadMetricsObserver

This CL adds possible connection reuse-related metrics to GWS
PageLoadMetricsObserver. This does not precisely measure the connection
reuse success / failure rate, but will give us insight on possible
candidates of these cases.

We will be adding a more precise metrics in future CL, but for the time
being.

NO_IFTTT=This CL adds the check

Bug: 373477776
Change-Id: I1fed02c366ec6c9b42bf231fa41dcfe019612758
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5932486
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Commit-Queue: Keita Suzuki <suzukikeita@chromium.org>
Reviewed-by: Minoru Chikamune <chikamune@chromium.org>
Reviewed-by: Nidhi Jaju <nidhijaju@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1369810}