Add standard 5XX alert for load balancer

Description

Add a standard alert which is triggered by a set number of 5XX status codes from the load balancer being seen over a period of a minute.

Further details

This alert should:

• Be on by default
• Have configuration for:
  • Turning the alert on or off (default: on)
  • Time period to aggregate over (default: 1 minute)
  • Threshold for alert to trigger (default: 10)
  • The alert policy & condition names
    • Default: see https://gitlab.developers.cam.ac.uk/uis/devops/iam/authentication/infrastructure/-/issues/191
• Be present in the v9 and v8 branches, i.e. this change will require two merge requests

Task list

Acceptance criteria

• Cloud run webapp terraform module includes standard alert for 5XX responses
• Configuration is as described above - in particular this alert should be specifically configured on or off by itself (i.e. not tied to all alerts being enabled or not)

Links/references