Generate Request-Id header for spotlight

The networking is…interesting. We have:

```
GET /performance
Host www.gov.uk
```

over the public internet goes through CDN to GOV.UK origin servers.

Here it hits the [router](https://github.com/alphagov/router).

The router looks up data in the[router-api](https://github.com/alphagov/router-api) to decide which
backend handles that request. Currently this is the spotlight backend. It
then goes via a local `dnsmasq` | `/etc/hosts` lookup to the GOV.UK frontend
loadbalancer, where it is then proxied via ngxinx to the Performance
Platform environment.