Add summary statistics to the prometheus dashboard

What does this MR do?

Adds summary statistics (Max and Average) to the prometheus dashboard legends

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

closes #43973 (closed)

EE counterpart

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5235