Refactor topology spec

What does this MR do?

Address the discussion from !40971 (merged):

• @mkaeppler started a discussion: (+3 comments)

  I don't think you need any of these tests; you're testing the same behavior in multiple different places.

  Test boundaries should be established around interface contracts: UsageData has an interface contract with the Prometheus client API (and helpers) through with_prometheus_client. This is what should be stubbed here, not the underlying mechanics of Prometheus.

  You're just making your own life unncessarily difficult here :-) and it also creates test coupling, which is not good.

This MR stub with_prometheus_client in topology_spec.rb. So it avoids duplicating the tested behavior of with_prometheus_client in spec/lib/gitlab/utils/usage_data_spec.rb.

Conformity

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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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Closes #249136 (closed)