Gitlab::Metrics.current_transaction needs to be public for RailsQueueDuration

What does this MR do?

Change the scope of the module method current_transaction to be used on RailsQueueDuration middleware

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

Why was this MR needed?

Closes #20473 (closed)

What are the relevant issue numbers?

Screenshots (if relevant)

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 style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together