Resolve "Prometheus service page shows error"

What does this MR do?

Changes a $.json call to an $.ajax call to customize warnings as to not use global alerts and prevent our listener ($(document).ajaxError) in main.js to alert in case of an 404 not found request when no metrics are configured

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #35652 (closed)