WIP: Detect alert dashboard embeds in GFM

What does this MR do?

Adds functionality to the GFM pipeline to expose !23484 (merged) for usage on the FE. This will allow us to drop a url for a particular gitlab-defined firing alert into GFM & have a chart for the corresponding metric appear as an embed.

Embeddable path is currently of the format: /*namespace_id/:project_id/prometheus/alerts/:id/metrics_dashboard

Still missing:

• Redaction where embed should be unavailable to the user.
• Update to detection logic which should look for an empty span with a data attribute

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

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