Add internal routes for api and api-sms-receipts
Add internal routes for api and api-sms-receipts

These routes will be used by prometheus to scrape the `/metrics` endpoint.

Currently:

The shared prometheus scrapes the `/metrics` endpoint using
the public routes.

The `/metrics` endpoint is provided by the [gds_metrics_python][] which
comes with [bearer-token authentication][] where the token is expected
to be equal to the paas app id.

Each app is configured as a separate target in the shared prometheus
with its app id configured as a GET parameter (e.g.
http://notify-api-production.cloudapps.digital/metrics?cf_app_guid=69c87503-6b53-4c35-XXXX-XXXXXXXXXXXX&cf_app_instance=69c87503-6b53-4c35-XXXX-XXXXXXXXXXXX%3A1&cf_app_instance_index=1)

Each scrape request goes through an nginx proxy which retrieves this
argument from the query string and sets it as a header [[source][]]. This way it
passes the authentication and also is able to instruct the gorouter to
target a specific instance of the app.

In the future:

Since we're moving away from the shared prometheus and towards an
approach where we [run our own prometheus on PaaS][] we can skip the
need for having an nginx proxy and use the internal routes instead, and
have a [preshared-token][] for authentication if we need to.

[gds_metrics_python]: https://github.com/Crown-Commercial-Service/gds_metrics_python
[bearer-token authentication]: https://github.com/Crown-Commercial-Service/gds_metrics_python/blob/master/gds_metrics/__init__.py#L47-L52
[source]: https://github.com/alphagov/prometheus-aws-configuration-beta/blob/master/terraform/modules/prom-ec2/prometheus/cloud.conf#L111-L123
[run our own prometheus on PaaS]: https://github.com/alphagov/notifications-cf-monitoring/pull/1
[preshared-token]: https://github.com/Crown-Commercial-Service/gds_metrics_python/pull/18

alphagov/notifications-api,alphagov/notifications-api