Specify a dropdown name for dashboards

What does this MR do?

The metrics dashboard (Project > Operations > Metrics) is adding a dropdown to select project-defined dashboards (https://gitlab.com/gitlab-org/gitlab-ce/issues/59974). For the first iteration, the dropdown lists the full filepath. This is descriptive & complete, but is too much extra information. This MR provides a display name to the FE, including enough info to uniquely identify the dashboard, minus the cruft.

This is a backend only-change and won't actually change what the user sees, so a changelog probably isn't necessary.

Does this MR meet the acceptance criteria?

Conformity

• [n/a] Changelog entry
• [n/a] 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