Add regex to match Prometheus Alert endpoint for GFM identification

What does this MR do?

Adds a matcher for identifying a url as pointing to the Prometheus Alert #show endpoint.

This will be used from the GFM pipeline to automatically render metrics corresponding to an alert id. See #119016 (closed) for further detail.

This is not a user-facing change, so I won't include a changelog this time.

Also trying to reduce potential merge conflicts with !24601 (merged), which will be support for a very similar feature.

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

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