Track the position of reviewers selected from the MR reviewer sidebar

What does this MR do and why?

For #512710 (closed)

Adds two events and associated metrics:

1. User selects a reviewer in the MR reviewer sidebar
2. User selects a reviewer in the MR reviewer sidebar after searching/filtering the list

Each of these events has a value, which should contain the numerical position of the selected reviewer in the list.

MR acceptance checklist

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

Screenshots or screen recordings

This is all tracking events, backstage.

How to set up and validate locally

1. Have a Premium or Ultimate GDK
2. Go to an MR in a project with at least 2 users (3, if you want to exclude yourself)
3. In the MR sidebar Reviewers area, click "Assign"
   • 
4. Add a reviewer in the "Assign reviewers" sidebar
   • 
5. Blur / Close the reviewers dropdown
6. In your browser console's Network tab, confirm that the correct track_event network request is sent.
   • The events are user_selects_reviewer_from_mr_sidebar and user_selects_reviewer_from_mr_sidebar_after_search (if a search term was in the dropdown when you selected a reviewer)
   • The payload should match { ... "additional_properties": { "value": # } ... }, where # is the position of the reviewer you selected beginning after any other already-selected reviewers
   • In the following video, I select reviewer 1
   • Screencast_from_2025-02-06_21-54-54