fix: silence alarms in CODE and handle missing data correctly

[jacobwinch]

What does this change?

This PR makes two improvements to our alarms:

1. Disables alarm actions in CODE by default. This prevents us from spamming teams with alerts about their CODE environments (in my experience these just generate unactionable noise, as CODE environments are expected to break sometimes).
2. Handles missing data correctly for lambda-error-percentage alarms. No data in this scenario is fine; it just means that the lambda has not been invoked at all.

Does this change require changes to existing projects or CDK CLI?

No. Any user who wants to keep (or temporarily test) alarm actions in their CODE environment can still do so if they wish by setting actionsEnabledInCode to true.

How to test

I've tried upgrading https://github.com/guardian/tag-janitor/blob/main/cdk/lib/cdk-stack.ts (locally) in order to use this change and confirmed that the changes set looks sensible in AWS. I haven't tested actually deploying the change as there is no CODE environment for this stack. I'll finish the upgrade and check the deployment in PROD after releasing this change.

How can we measure success?

• We should receive fewer unactionable alarms
• Lambda-error-percentage alarms will be considered OK in scenarios where the lambda is not being invoked

Have we considered potential risks?

There is a small risk that disabling these alarm actions in CODE (by default) will prevent us from noticing problems with alarm configuration. I think using the ActionsEnabled property allows us to minimise this risk as much as possible.

[akash1810]

Excellent improvement 👍🏽

[github-actions]

🎉 This PR is included in version 8.0.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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