Use snake_case for metrics data and save them in a fixture

What does this MR do?

Use snake_case for metrics data and save them in a fixture

Frontend usually expects data in camelCase, and our tooling mostly considers that format. All things being equal, is best to use snake_case for HTML data.

Currently the initial state and the initial props overlap in function.

As the monitoring app evolves we should check the input for it is stable. This change makes the separation of props and state predictable and verified with a snapshot.

Depends on !33301 (merged)

Screenshots

N/A

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

Closes #215316 (closed)