DCMAW-82345 create activeSession APIgw alarms

[sandymay-dd]

​DCMAW-8234

What changed

A low and high threshold alarm on both the 4XX and 5XX GET /async/activeSession endpoint

Testing

4XX: Invoked the endpoint with a badly formed request which results in a 4XX response.

5XX: Deployed a stack updating lambda handler typescript to return a 500.

How to review

A static check that the alarm configuration matches the requirements in the ticket.
Review the warning runbooks: https://govukverify.atlassian.net/wiki/spaces/DCMAW/pages/4800446694/Alarms+Runbook+Warnings
Review the critical runbooks: https://govukverify.atlassian.net/wiki/spaces/DCMAW/pages/4799594677/Alarms+Runbook+Alerts+-+DRAFT

Checklists

• There is a ticket raised for this PR that is present in the branch name
• No PII data logged. See guidance here
• Demo to a BA, TA, and the team.
• Update README with any new instructions or tasks

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud