Migrate vagov-heartbeat

[pjhill]

Issue Description

vagov-heartbeat canary needs to move out of AWS Cloudwatch and into Datadog.

---

Tasks

• [x] Analyze the Cloudwatch version of the synthetic
  • Consider PagerDuty alerts
  • Consider Slack notifications
  • Consider schedule
• [x] Re-create the behavior of the Cloudwatch version in Datadog
• [x] Monitor the new Datadog monitor for any unexpected failures

Acceptance Criteria

• [x] Datadog synthetic monitor alerts PagerDuty when meeting fail threshold

• [ ] Datadog synthetic monitor is included in a synthetic monitor dashboard

[ddzz]

We've reached our quota for browser tests in Datadog, so I can't create a new monitor for this canary.

[ddzz]

Monitor has been created: https://app.datadoghq.com/synthetics/details/jsd-8pg-2hq

[ddzz]

This monitor runs once every five minutes. It is retried once if it fails. #platform-qa-standards will be notified of any failures. I will monitor this test for the next few days for any unexpected failures.