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The number of in-flight requests can tell the observed queueing delay in
the next few seconds. The higher count of in-flight requests would cause
a high network queueing delay. Only tracks the peak values to eliminate
the noises in the data.

Adds methods to track the mapping between the peak number of in-flight
requests to the peak observed queueing delay within a measurement
period.

Change-Id: Id7093ffe65ad6cb5f7a0d050a403040d4bda9fa3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1700280
Commit-Queue: Jianfeng Wang <jfwang@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678052}