Add support for metrics embeds not backed by the DB

What does this MR do?

This MR adds support for metrics embeds to be defined & rendered entirely on the fly, rather than stemming from data in the DB.

This is to enable embedding of metrics based on alerts coming from a manually configured prometheus instance. As these alerts aren't associated with any alerting rules we manage for the user, we don't have record of the alert at the time it begins firing. So this embed type will allow the embed to be defined in GFM as a part of the /metrics_dashboard query params.

This has a disadvantage of a potentially quite long URL - #213276.

Issue: #195739 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required) - Skipping because this isn't actually intended to be used for users
• 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