Debug and Verify that EBS disk size alerts go through to Slack channel

[sgibson91]

Context

In #5004, we set up monitoring and alerting for EBS disk size in AWS that have the per-user quota feature enabled. We verified that an alert from Prometheus alertmanager triggered an incident in PagerDuty, but for some reason this didn't get automagically surfaced up to the #pagerduty-notifications channel in Slack. We should figure out why.

Task list

Tasks

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1. Figure out what needs to happen so that prometheus/alertmanager/pagerduty incidents about EBS disk size are propagated through to the #pagerduty-notifications Slack channel and implement it

Definition of Done

• When an EBS volume disk full alert is triggered, a message is sent to the #pagerduty-notifications channel in Slack