Add server request processing time logging
Add server request processing time logging

Summary:
Pretty straightforward stuff.
In a meeting a 1 - 2 weeks back, i think Mihir and Vishal said this
is necessary. I am unsure if it still is, but am arc diffing this as a first
time I touch this project. If unnecessary, will abandon.
I don't know where these stats go. Basically followed Mihir's diff
(https://tails.corp.dropbox.com/D109270)
Notes:
1. Maybe we need different keys, though the endpoint name should be unique enough
2. Unsure if this captures everything we care for, but to capture the entire
   process, I need to hack into the server framework (flask?). This would make it hard
   to upgrade the packages, so I figured, maybe this is good enough

Test Plan: Played with my local changes setup, and it didn't ruin anything.

Reviewers: haoyi, vishal, mkedia, ptyl

Subscribers: wwu, changesbot, mkedia, tabbott

Differential Revision: https://tails.corp.dropbox.com/D114605

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