Add alert for excessive DB time of security policy workers

Alerts when a groupsecurity policies worker spends excessive time in the database (gitlab-com/gl-infra/scalability#2674 (closed))

Compare with the Worker Detail/Average Database Time panel for our workers during the recent incident on Nov 22nd.

• 6h time range during incident:

  

  thanos-query.ops.gitlab.net

Resolves gitlab-com/gl-infra/scalability#2674 (closed)