fix(cloudwatch): Math:UnknownIdentifier warning for INSIGHT_RULE_METRIC (#28870)

The CloudWatch `MathExpression` class warns about identifiers missing from `usingMetrics` when `INSIGHT_RULE_METRIC` is used in the expression. It incorrectly parses the arguments to `INSIGHT_RULE_METRIC` as identifiers. When using `INSIGHT_RULE_METRIC`, I don't believe there is anything that needs to be added to `usingMetrics`.

This implementation follows a similar fix done for some other expressions here: #24313

----

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

Author: aws-sde

packages/aws-cdk-lib/aws-cloudwatch/lib/metric.ts

@@ -625,7 +625,7 @@ export class MathExpression implements IMetric {
     const missingIdentifiers = allIdentifiersInExpression(this.expression).filter(i => !this.usingMetrics[i]);
 
     const warnings: { [id: string]: string } = {};
-    if (!this.expression.toUpperCase().match('\\s*SELECT|SEARCH|METRICS\\s.*') && missingIdentifiers.length > 0) {
+    if (!this.expression.toUpperCase().match('\\s*INSIGHT_RULE_METRIC|SELECT|SEARCH|METRICS\\s.*') && missingIdentifiers.length > 0) {
       warnings['CloudWatch:Math:UnknownIdentifier'] = `Math expression '${this.expression}' references unknown identifiers: ${missingIdentifiers.join(', ')}. Please add them to the 'usingMetrics' map.`;
     }
 

packages/aws-cdk-lib/aws-cloudwatch/test/metric-math.test.ts

@@ -100,6 +100,15 @@ describe('Metric Math', () => {
     expect(m.warningsV2).toBeUndefined();
   });
 
+  test('metrics INSIGHT_RULE_METRIC expression does not produce warning for unknown identifier', () => {
+    const m = new MathExpression({
+      expression: "INSIGHT_RULE_METRIC('RejectedConnectionsRule', 'Sum')",
+      usingMetrics: {},
+    });
+
+    expect(m.warningsV2).toBeUndefined();
+  });
+
   test('math expression referring to unknown expressions produces a warning, even when nested', () => {
     const m = new MathExpression({
       expression: 'e1 + 5',