Add alerts for a spike or increased calls to /users/sign_in

As part of the incident review OnCall report for period: 2019-05-28 - 2019-06-04 (gitlab-com/gl-infra/production#867 - closed) it was suggested that we should have alerts on the /users/sign_in endpoint as a corrective action . We can start with a 25% increase in the number of calls to the endpoint from the average daily number.