fix(cloudwatch): update regex expression that prevents CloudWatch:Mah:UnknownIdentifier warnings (#33591) (#33592)

### Issue # (if applicable)

Closes #33591

### Reason for this change



* The validation is causing false-positive warning messages when using partial identifiers on CloudWatch Metric expressions supporting them.

### Description of changes

* Update the condition to generate the warning message in order to prevent false-positives when one of the special keywords is used.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

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

Author: garciparedes

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

@@ -707,7 +707,7 @@ export class MathExpression implements IMetric {
     // we can add warnings.
     const missingIdentifiers = allIdentifiersInExpression(this.expression).filter(i => !this.usingMetrics[i]);
 
-    if (!this.expression.toUpperCase().match('\\s*INSIGHT_RULE_METRIC|SELECT|SEARCH|METRICS\\s.*') && missingIdentifiers.length > 0) {
+    if (!this.expression.toUpperCase().match('\\b(INSIGHT_RULE_METRIC|SELECT|SEARCH|METRICS)\\b') && 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

@@ -80,6 +80,15 @@ describe('Metric Math', () => {
     expect(m.warningsV2).toBeUndefined();
   });
 
+  test('metrics METRICS expression with parameter does not produce warning for unknown identifier', () => {
+    const m = new MathExpression({
+      expression: 'SUM(METRICS("parameter"))',
+      usingMetrics: {},
+    });
+
+    expect(m.warningsV2).toBeUndefined();
+  });
+
   test('metrics search expression does not produce warning for unknown identifier', () => {
     const m = new MathExpression({
       expression: "SEARCH('{dimension_one, dimension_two} my_metric', 'Average', 300)",