Add alert for increase in active sessions

as a corrective action for production#553 (closed) we should add an alert for a large increase in active sessions.

https://dashboards.gitlab.net/d/wyBYDjnmk/gitlab-workhorse?panelId=8&fullscreen&orgId=1&from=1541446106294&to=1541526372156

Screen_Shot_2018-11-06_at_6.46.53_PM