Add documentation
Remove template parameter

I think this is leftover from when we changed the nginx module.

See ac5714d8.
Validate parameters

It’s a good idea, and leads to easier maintenance in the future.
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.
Merge pull request #600 from alphagov/feature/spotlight-request-id

Generate Request-Id header for spotlight