Add a monitor for cloud controller unregistrys
Add a monitor for cloud controller unregistrys

During a recent incident we had a monitor which overloaded the API.

Once the API's performance fell below a certain point it started failing
its healtcheck and getting unregistered from gorouter.

This commit adds monitoring so we'll be alerted if the rate of API
unregistrations over 1 hour increases above a threshold.

The thresholds we've chosen here were picked so that they would not warn
under average conditions but would have been critical during the incident

See https://app.datadoghq.com/notebook/63780/Cloud%20Controller%20outage%20metrics

alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf,alphagov/paas-cf