Resolve "Measure multi-file editor usage on GitLab.com"

This adds a metric counter, to capture:

• log commit size (number of files changed, number of lines changed) of commits from multi-file editor
• log anonymous project identifier to determine adoption by project
• log anonymous user identifier to determine adoption by user

---

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Closes #4547 (closed)