Refactor custom metrics form fields test spec using new axios helper

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The following discussion from !11393 (merged) should be addressed:

• @winh started a discussion:

  I think this works as a temporary solution. Can we create a follow-up issue to refactor this after https://gitlab.com/gitlab-org/gitlab-ce/issues/60972 has been solved?

Files needing refactored (and their corresponding merge request)

• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/11393 ee/spec/frontend/custom_metrics/components/custom_metrics_form_fields_spec.js
• (not yet merged) https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28431 spec/frontend/operation_settings/components/external_dashboard_spec.js