Add "request time" event and viewer and context data to Multimeter

Summary: Ref T6930. Only notable thing here is that I prevented non-admins from slicing down by viewing user, since it feels a little creepy to go see what pages you looked at, even though we only show which controllers you invoked. However, it feels important enough to be able to see users destorying the server with crazy requests to let admins see this data.

Test Plan:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6930

Differential Revision: https://secure.phabricator.com/D12630