R1:7b6c320e156a

Skeleton for "Multimeter", a performance sampling application

Summary:
Ref T6930. This application collects and displays performance samples -- roughly, things Phabricator spent some kind of resource on. It will collect samples on different types of resources and events:

- Wall time (queries, service calls, pages)
- Bytes In / Bytes Out (requests)
- Implicit requests to CSS/JS (static resources)

I've started with the simplest case (static resources), since this can be used in an immediate…