Define separate layer for prometheus requests

What does this MR do?

Refactor in preparation for #228760 (closed)

As part of our frontend will be relatively "stateless" in the new metric builder, not all of our data will be fetched from the store actions.

This MR creates a new requests/index.js file to place the lower level function to access data, so the backoff is abstracted from the store.

Additionally, the error handling was updated to match the prometheus error responses.

Before

• actions.js -> axios

After

• actions.js -> metrics_requests.js -> axios
• <new builder component or service> -> metrics_request.js -> axios

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