Tune thresholds for sidekiq exception counts

Closes gitlab-com/gl-infra/infrastructure#5201

It'd be cool if these alerts pointed to a runbook. If either of these alerts fire, at present, I have no idea what to do.

approved this merge request

added 2 commits

• 883f6ac5 - 1 commit from branch master
• 08f3fc5c - Tune thresholds for sidekiq exception counts

Compare with previous version

@skarbek - @dsylva added a skeleton runbook in !767 (merged), and I have a todo to update it with troubleshooting info (see gitlab-com/gl-infra/infrastructure#5203)

merged