Change Prometheus metrics names

What does this MR do?

Implements metrics rename as discussed in #3114 (closed).

Why was this MR needed?

To "brand" metrics with gitlab_runner_ prefix (instead of ci_), to make names more descriptive, and to make metrics names and implementation more aligned with Prometheus community standards.

Are there points in the code the reviewer needs to double check?

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?

Closes #3114 (closed)