Process prometheus-formatted alerts via HTTP integrations

What does this MR do?

Related issue: #255501 (closed)

If an alert sent to the legacy generic alert endpoint could be processed as a prometheus alert, it would be. With the migration to HTTP endpoints, this behavior stayed the same for already-existing integrations. For new HTTP integrations, this is not the case.

The MR brings the same process-as-prometheus functionality to alerts sent through HTTP integrations.

Screenshots

Test alert sent to HTTP endpoint	Processed alert details view

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