Check for minimal count of performance bar metrics

What does this MR do?

This test sometimes fails because the 'total' metric is not always displayed for a page request, causing the assertion that 4 metrics are displayed to be false. This has been observed on Gitlab.com, but the cause isn't currently known. This behavior may not be new but is surfacing now because we recently changed the assertion to specify the number of elements to appear.

This MR asserts the minimum number of metrics to appear.

Closes #197135 (closed)

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines