Fix `tracking_label` usage [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

This does:

• Fixes a ~bug introduced by !51924 (merged)
• Ensures that tracking_label can be called without a subject
• Ensures that code using this method properly calls experimentation framework.

This is needed to solve the QA failure that was found on staging. Where a number of requests resulted in 500: https://nonprod-log.gitlab.net/goto/2c42db7a39f29b3167cd85b42fe2a167

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides