Refactor MetricsHook unit tests to use new MetricCollector pattern

[askpt]

Description

The new unit test pattern introduced in #512 (using the MetricCollector approach for MetricsHook unit tests) is clearer and easier to test. For consistency, this work should be replicated in other unit tests.

Acceptance Criteria:

• Refactor existing MetricsHook-related unit tests to use the new MetricCollector approach, following the pattern introduced in feat: Add Metric Hook Custom Attributes #512.
• Ensure all refactored tests retain their previous coverage and intent.
• Do not change the logic of the tests, only the test structure and pattern.
• Add/update documentation if necessary.

References:

• PR: feat: Add Metric Hook Custom Attributes #512
• Discussion: feat: Add Metric Hook Custom Attributes #512 (comment)

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Comment context:

This seems to be using a different pattern. I think it makes more sense, when you compare with the previous implementation. If we merge this as it is, I would advice to replicate this pattern to the other unit tests.

Originally posted by @askpt in #512 (comment)