metrics: Validate parameters for a Metric.dimensions

[cybergoof]

• I'm submitting a ...

  • 🪲 bug report
  • [ x ] 🚀 feature request
  • 📚 construct library gap
  • ☎️ security issue or vulnerability => Please see policy
  • ❓ support request => Please see note at the top of this template.

• What is the current behavior?
  When creating a new Metric for a CloudWatch dashboard, an 'undefined' is accepted for the dimensions value, but CloudFormation will reject it.

• What is the expected behavior (or behavior of feature suggested)?
  CDK should evaluate the name/value of a dimension and reject invalid values.

• What is the motivation / use case for changing the behavior or adding this feature?
  Input validation at the CDK build process is better than waiting for CloudFormation.

• Please tell us about your environment:

  • CDK CLI Version: 0.35.0
  • Module Version: 0.35.0
  • OS: [all ]
  • Language: [TypeScript]

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. associated pull-request, stackoverflow, gitter, etc)

        const dashboard = new cloudwatch.Dashboard(this, name, { dashboardName: name});
        const newMetric = new Metric({
            dimensions: {
                AutoScalingGroupName: undefined,
            },
            metricName: "CPUUtilization",
            namespace: "AWS/EC2",
            statistic: "avg",
        });
        dashboard.addWidgets(
            new GraphWidget({
                left: [
                    newMetric,
                ],
                title: `Average CPU`,
           }),
        );
    }

The above will build. But the following error is given from CloudFormation

The dashboard body is invalid, there are 1 validation errors: [ { "dataPath": "/widgets/0/properties/metrics/0", "message": "Should NOT have more than 3 items" } ] (Service: AmazonCloudWatch; Status Code: 400; Error Code: InvalidParameterInput; 

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