Update metric dashboard validator

What does this MR do?

For &3971

!38237 (merged) added a new column dashboard_path to PrometheusMetrics. This MR uses that new column to validate prometheus_metric identifier uniqueness. Currently dashboard_path is nil for all PrometheusMetrics. We will start populating it once we start importing custom metrics into the database.

Nothing actively uses this code yet.

Screenshots

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