Create service and endpoint for embedding metrics by alert

What does this MR do?

This MR moves us one step closer to the ability to automatically embed a metric chart in an incident issue based on the alert that is firing.

Parts of this MR:

• New metrics dashboard endpoint which can surface an embeddable dashboard corresponding to a given alert id
• New dashboard service which finds & formats a metric as an embeddable dashboard
• Makes new service discoverable in the dashboard ServiceSelector

Out of scope:

• Recognition of new dashboard endpoint in GFM pipeline (future MR - !23490 (closed))
• Pulling embed formatting fields from the yml sources (future MR/possibly future issue. Requires some thinking still...)
• Alert rules which aren't created through GitLab (future issue - #195739 (closed))

Related Issue: #119016 (closed)

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