Add nb_profilable metric counting profilable visits. #19745

Description:

Refs #16363

I'm finishing up #16773, and thought it would be better to review/merge it piecemeal. This PR adds nb_profilable as a metric to VisitsSummary.get. It's just the sum of all profilable visits, and will be used in further PRs.

Review

• [ ] Functional review done
• [ ] Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done
• [ ] Wording review done
• [ ] Code review done
• [ ] Tests were added if useful/possible
• [ ] Reviewed for breaking changes
• [ ] Developer changelog updated if needed
• [ ] Documentation added if needed
• [ ] Existing documentation updated if needed

This issue is in "needs review" but there has been no activity for 7 days. ping @matomo-org/core-reviewers