Add cache-metrics to /builds/register.json

What does this MR do?

Adds Cache Metrics to /builds/register.json

Why was this MR needed?

We don't know if the cache is hit or not

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

!8084 (merged)