Add rule and alert for SAML login errors

This MR adds alert for detecting SAML login request errors (500 errors). We recently had an incident where the error rate for a particular controller increased due to an application error (inefficient DB query). To quickly detect similar problems in the future, we'd like to add an alert for it.

• Related incident: gitlab-com/gl-infra/production#17856 (closed)
• Incident review: gitlab-com/gl-infra/production#17863 (closed)

added typemaintenance label

assigned to @ahegyi

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Thanos query:

sum by (env,environment,stage) (sli_aggregations:gitlab_sli_rails_request_error_total_rate1m{job="gitlab-rails", endpoint_id="Groups::OmniauthCallbacksController#group_saml", type= "web"} offset 30s)

added 1 commit

• b6c5957c - Add rule and alert for SAML login errors

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Hi, @tmillhouse! Could you review this MR?

requested review from @tmillhouse

changed the description

approved this merge request

resolved all threads

changed milestone to %17.0

@stejacks-gitlab, could you review and merge this MR?

requested review from @stejacks-gitlab

mentioned in issue gitlab-com/gl-infra/production#17863 (closed)

added this merge request to the merge train at position 4

resolved all threads

removed review request for @stejacks-gitlab

merged

mentioned in commit eb964806