fix(cloudwatch): render region and accountId when directly set on metrics

[TrevorBurnham]

Issue

Closes #28731

Reason for this change

Currently, if a user creates a metric that includes region and accountId, those fields are omitted when the metric renders in a stack with those same values. The intended behavior is to omit those fields when they're implicitly set via metric.attachTo(stack), not to omit them when set directly by the user.

This change differentiates those behaviors and adds a test case.

Description of changes

The key changes here are on the Metric class. Previously, region and account were public properties that were set by metric.attachTo(stack), making it impossible to differentiate whether they were set directly or via attachTo. To differentiate them while maintaining backward compatibility, I took this approach:

1. attachTo sets internal properties called stackRegion and stackAccount. Setting region and account directly sets internal properties called regionOverride and accountOverride.
2. The public region and account properties are now getters that return the override (if set) and fall back on the stack properties.
3. The toMetricConfig() method returns the region and account from the getters, but also includes the regionOverride and accountOverride.

That way, everything that looks at region and account works the same way it did before, except in metricGraphJson, which skips the "if different from stack" tokenization if the overrides are set.

Description of how you validated changes

All existing unit tests are satisfied by these changes.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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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

[aws-cdk-automation]

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[TrevorBurnham]

The PR linter says "Fixes must contain a change to an integration test file and the resulting snapshot." Is there an existing integration test that's relevant to this functionality? I'm happy to add one, but I could use some direction.

[aws-cdk-automation]

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[aws-cdk-automation]

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