Make Prometheus metrics endpoint return empty response when metrics are disabled

What does this MR do?

Instead of displaying normal 404 page we display empty metrics response with short information that metrics are disabled and a link to documentation showing how it can be turned on.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes: #35639 (closed)