(aws-cloudwatch): Sort visible metrics first in widgets

### Describe the feature

Change the behavior of how metrics are sorted in CloudWatch Dashboard Widgets so that visible metrics appear first.

### Use Case

Using multiple `MathExpression`s in the same widget leads to difficult to decipher graphs, because the colors of visible metrics are very similar. See the screenshot for an example: in view, there are 3 visible metrics, but because there are many hidden metrics between the visible ones, the automatic colors chosen aren't easily distinguishable.

### Proposed Solution

Before rendering the dashboard json of a widget, sort the metrics by visible first. This would make CloudWatch's automatic color scheme work much better for the metrics that actually appear in the widget.

### Other Information

*Note: I do know that defining the colors manually is possible, but this is not a good solution for a few reasons. Namely, I don't always know how many metrics will be in a widget and I would prefer not to re-implement the automatic color generation how CloudWatch does it*

I've created a sample dashboard in [this repository](https://github.com/jackrein/aws-cdk-cloudwatch-math-expression-widget) to demonstrate this behavior.

### Acknowledgements

- [x] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### AWS CDK Library version (aws-cdk-lib)

aws-cdk-lib@2.215.0

### AWS CDK CLI version

2.1031.2 (build 779352d)

### Environment details (OS name and version, etc.)

Amazon Linux 2

No implementation file or test is named. Start by locating the TypeScript CloudWatch dashboard widget code that renders dashboard JSON, then trace how metric visibility is represented and ordered; done means visible metrics precede hidden metrics without changing the widget's other output.

Written by the indexing model from the issue text.