fix(cloudwatch): math expressions incorrectly warn about search and metrics (#24313)

Closes [#20136](#20136).

It is intended that all metric identifiers referenced in a MathExpression are included in the usingMetrics map and we will raise warnings if the customer does not follow this contract. 
However for SEARCH and METRICS queries, we can refer directly to metrics attribute values inside the query. Therefore we should not raise warnings.

Change made based on work done in 55108b9 with regex extended to a few other expressions. Looks like integ requests will not be required based on that commit.

I had some firsthand experience getting thousands of this warning message after upgrading to CDK 2 and decided it would be easier to fix than suppress the excessive warnings.


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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*

Author: ParkerVR

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

@@ -609,9 +609,9 @@ export class MathExpression implements IMetric {
     // we can add warnings.
     const missingIdentifiers = allIdentifiersInExpression(this.expression).filter(i => !this.usingMetrics[i]);
 
-    const warnings = [];
+    const warnings: string[] = [];
 
-    if (!this.expression.toUpperCase().match('\\s*SELECT\\s.*') && missingIdentifiers.length > 0) {
+    if (!this.expression.toUpperCase().match('\\s*SELECT|SEARCH|METRICS\\s.*') && missingIdentifiers.length > 0) {
       warnings.push(`Math expression '${this.expression}' references unknown identifiers: ${missingIdentifiers.join(', ')}. Please add them to the 'usingMetrics' map.`);
     }
 

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

@@ -75,6 +75,24 @@ describe('Metric Math', () => {
     expect(m.warnings).toContainEqual(expect.stringContaining("'m1 + m2' references unknown identifiers"));
   });
 
+  test('metrics METRICS expression does not produce warning for unknown identifier', () => {
+    const m = new MathExpression({
+      expression: 'SUM(METRICS())',
+      usingMetrics: {},
+    });
+
+    expect(m.warnings).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)",
+      usingMetrics: {},
+    });
+
+    expect(m.warnings).toBeUndefined();
+  });
+
   test('metrics insights expression does not produce warning for unknown identifier', () => {
     const m = new MathExpression({
       expression: "SELECT AVG(CpuUsage) FROM EC2 WHERE Instance = '123456'",