feat(cloudwatch): validate Dashboards with an end time must also have a start time (#27124)

This PR adds a validation for a `start` and an `end` in `Dashboard`.

It throws an error if you specify an `end` without a `start`.

While it is possible to deploy a dashboard with only an end time in CloudFormation, the setting will be ignored and the dashboard displayed with the default time period.

With this validation, we are stopping the faulty deployment early.
API Reference contains the following description and the user will not be aware of this situation.

If you specify a value for end, you must also specify a value for start.

https://docs.aws.amazon.com/AmazonCloudWatch/latest/APIReference/CloudWatch-Dashboard-Body-Structure.html

And each Widget in graph.ts also validates the same case.

https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/aws-cloudwatch/lib/graph.ts#L245-L247

----

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

Author: go-to-k

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

@@ -133,7 +133,11 @@ export class Dashboard extends Resource {
     }
 
     if (props.start !== undefined && props.defaultInterval !== undefined) {
-      throw ('both properties defaultInterval and start cannot be set at once');
+      throw new Error('both properties defaultInterval and start cannot be set at once');
+    }
+
+    if (props.end !== undefined && props.start === undefined) {
+      throw new Error('If you specify a value for end, you must also specify a value for start.');
     }
 
     const dashboard = new CfnDashboard(this, 'Resource', {

packages/aws-cdk-lib/aws-cloudwatch/test/dashboard.test.ts

@@ -161,6 +161,35 @@ describe('Dashboard', () => {
 
   });
 
+  test('throws if both defaultInterval and start are specified', () => {
+    // GIVEN
+    const stack = new Stack();
+    // WHEN
+    const toThrow = () => {
+      new Dashboard(stack, 'Dash', {
+        start: '-P7D',
+        defaultInterval: Duration.days(7),
+      });
+    };
+
+    // THEN
+    expect(() => toThrow()).toThrow(/both properties defaultInterval and start cannot be set at once/);
+  });
+
+  test('throws if end is specified but start is not', () => {
+    // GIVEN
+    const stack = new Stack();
+    // WHEN
+    const toThrow = () => {
+      new Dashboard(stack, 'Dash', {
+        end: '2018-12-17T06:00:00.000Z',
+      });
+    };
+
+    // THEN
+    expect(() => toThrow()).toThrow(/If you specify a value for end, you must also specify a value for start./);
+  });
+
   test('DashboardName is set when provided', () => {
     // GIVEN
     const app = new App();