Support loading metrics dashboard with file name

Background

Add Vue-router to metrics dashboard and support Vanity URLs. #214282 (closed)

What does this MR do?

The metrics dashboard requires the entire custom dashboard file path to work. This MR enables the dashboard to work with dashboard file names

Custom dashboards can now be accessed with just the file name this way

/metrics?dashboard=.gitlab%2Fdashboards%2Fmany_links.yml -> /metrics?dashboard=many_links.yml

Default dashboard will continue to entire path

/metrics?dashboard=config/prometheus/common_metrics.yml -> /metrics?dashboard=config/prometheus/common_metrics.yml

Note

The target branch is another branch that fixes some inconsistencies with the usage of getters in mutations. !34237 (merged)

Screenshots

metrics_dashboards_file_names

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team