Setup BE for real user FE perf monitoring

Frontend is building 2 types of perf monitoring.

For one of the monitoring tools (real user clientside performance) we need some backend to accompany it.

In gitlab-ce!10570 I use prom.js (a JS prometheus metrics registry) to introduce a system which will aggregate clientside and POST them as a string to some endpoint continuously at a given interval (currently every 15s).

This 'endpoint' needs to be a prom-aggregation-gateway which is then to be scraped by prometheus itself.