Make metrics dashboard endpoint embedded check more explicit

What does this MR do?

When a metrics dashboard is requested, the key :embedded in the query params is used to determine whether a request is for an embed or a full dashboard. Instead of relying on just the presence of the param, this swaps the check over to whether the param's value represents true/false.

This shouldn't result in any sort of change in behavior from internal callers of the endpoint, but it is technically a change in the internal API.

This is a follow-up from:

• !23484 (comment 282799153)
• !23484 (comment 280256265)

Screenshots

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