[ShussMiao]

Issue number: closes #8673

Summary

Changes

CloudWatchAlarmMetric.metric_stat read self["metricStat"] directly, so it raised KeyError: 'metricStat' on the expression entries of a metric math alarm. expression and label in the same class already use self.get(...) for optional keys, so this makes metric_stat consistent with them and widens the return type to CloudWatchAlarmMetricStat | None.

The added test uses the repository's existing cloudWatchAlarmEventSingleMetric.json fixture — its first metric (e1, expression: "m1/m2") has no metricStat — so no synthetic payload is needed.

Note on triage: this issue is not labelled triage yet, so flagging it here. The failure is reproducible from the repo's own event fixture with no AWS access, and origin/develop still reads self["metricStat"] at line 94 of the file.

User experience

Looping over alarm_data.configuration.metrics and reading metric_stat now works for metric math alarms instead of failing on the first expression entry.

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[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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.95%. Comparing base (26deef8) to head (eeeac64).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8691   +/-   ##
========================================
  Coverage    96.95%   96.95%           
========================================
  Files          321      321           
  Lines        16253    16254    +1     
  Branches      1453     1453           
========================================
+ Hits         15758    15759    +1     
  Misses         353      353           
  Partials       142      142           

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[leandrodamascena]

Thanks @ShussMiao, and welcome. I did not stop at the fixture: I created a real metric math alarm with a Lambda action, forced it into ALARM, and the event CloudWatch delivered has the expression entry without metricStat, exactly like the fixture. With your change the loop over metrics works and the two real metrics still resolve. Returning None keeps metric_stat consistent with expression and label. Approving, this goes out in 3.39.0, no rebase needed, and you'll be credited in the release notes.