Consider adding the Server Timing headers to GitLab responses

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Server timing abstract:

This specification enables a server to communicate performance metrics about the request-response cycle to the user agent. It also standardizes a JavaScript interface to enable applications to collect, process, and act on these metrics to optimize application delivery.

– http://wicg.github.io/server-timing/

There's already a gem that adds the support for Server Timing in Rails: https://github.com/dpogue/rails_server_timings

This could be related to gitlab-ce#29010 (e.g. we could display page timings via JS) even though https://github.com/peek/peek seems to have more data available.