Add Grafana to Admin > Monitoring menu when enabled

What does this MR do?

If Grafana is enabled, display a menu in admin settings > monitoring part to access it.

EE Backport: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14503

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Closes #61005 (closed)