chore: update error message to include key name for metric (#34221)

### Issue # (if applicable)

Closes #34156 

### Reason for this change

Updating the message to provide better message

### Description of changes

Simple change on the message to include key name

### Describe any new or updated permissions being added

N/A

### Description of how you validated changes

Local build has been successful

### Checklist
- [ ] 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)

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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: Y-JayKim

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

@@ -591,7 +591,7 @@ export class Metric implements IMetric {
 
     dimsArray.map(key => {
       if (dims[key] === undefined || dims[key] === null) {
-        throw new cdk.UnscopedValidationError(`Dimension value of '${dims[key]}' is invalid`);
+        throw new cdk.UnscopedValidationError(`Dimension value of '${dims[key]}' is invalid for key: ${key}`);
       }
       if (key.length < 1 || key.length > 255) {
         throw new cdk.UnscopedValidationError(`Dimension name must be at least 1 and no more than 255 characters; received ${key}`);