Move create_element to parent class for metric embeds

What does this MR do?

For metric filters the banzai pipline inserts a div in the html rendered markdown that includes a url to the dashboard data. The front end recognizes this div by a unique class name and makes a request to get the dashboard data. This moves some duplication in the parent class to ensure consistency across classes.

It is important that the div class name be consistent across classes because the redactor InlineMetricsRedactorFilter handles permissions to see the unfurled chart based on the div class name as well.

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

Part of #207439 (closed)