fix: make anomaly detection alarm work on math expression

[Laxenade]

Fixes #425
Fixes #340

Changes

Changes to the implementation of AnomalyDetectionMathExpression:

• Fixed an issue where thresholdMetricId was assigned the incorrect expression id when multiple math expressions were present in the generated CFN template.
• Modified returnData to be true only for the ANOMALY_DETECTION_BAND function and its direct dependency, rather than for all of the metrics in Metrics.

I haven't come across any internal or external documentation indicating that two returnData: true are necessary for anomaly detection. However, after doing some experiments on CFN, this appears to be true.

Testing

I copied the fix into my own CDK package, with the fix, I was able to deploy my stacks.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[echeung-amzn]

Thanks!