I used custom headers in the past to send some metrics over to the client–mostly for debugging purposes to quickly differentiate between client and server issues on slow web request and/or web application performance.
While that was working out pretty well I like standards and even if I did not tell anybody I always wanted such a thing built into HTTP.
A W3C standard is on the way: Server Timing
It consists of a spec how the HTTP Header is constructed:
Of course not all browsers support it yet
But Chrome 65–at the time of writing in the beta channel–does so: