Docs: fix test references in Metrics utility

[leandrodamascena]

What were you searching in the docs?

The PR #5160 moved the location of some of the test files and broke some links, we need to fix that.

Is this related to an existing documentation section?

https://docs.powertools.aws.dev/lambda/python/latest/core/metrics/

How can we improve?

Fix the links

Got a suggestion in mind?

No response

Acknowledgment

• I understand the final update might be different from my proposed suggestion, or refused.

[github-actions]

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