Add log message for experiment struct tracking events

What does this MR do and why?

Add log message for struct tracking events to verify lightweight_trial_registration_redesign is tracked correctly.

How to set up and validate locally

• Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK
• Sign out. Visit /-/trial_registrations/new.
• Check log/application_json.log.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #561744