[Preconnect] Add metrics related to PreresolveJob queueing

This CL adds metrics to help us gain insight into PreconnectManager's
queueing of requested preresolve jobs:

- The amount of time that a PreresolveJob spends queued
- The length of the queue at the time that PreconnectManager processes
  it

These metrics will help us gain insight into whether and how to
experiment with changing PreconnectManager's logic for when to stop
processing jobs from the queue in a given iteration.

Bug: 1257582
Change-Id: I8dbae69f72ef440cf80c7b1f3dfea56fea75d667
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3528742
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Egor Pasko <pasko@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#983728}