Fix alert creation dropdown menu

What does this MR do?

This MR resolves https://gitlab.com/gitlab-org/gitlab-ee/issues/12572. Monitor is in the process of switching over the endpoint that the metrics dashboard uses to populate data. As a part of this switch, a new API response format is being used. The particular problem in the issue is that the location of metric ids changed between APIs. This update allows for both response formats to be supported simultaneously.

This bug is still behind a feature flag which is off, so it's not yet user-facing. Excluding a changelog.

Does this MR meet the acceptance criteria?

Conformity

• [n/a] Changelog entry
• [n/a] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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