Adding a metric without a definition doesn't fail `every_metric_definition_spec.rb` specs

What does this MR do and why?

Related to #366844 (closed)

We're adding a fix to one of our tests, as we found an edge case for which it didn't catch issues [explained further in validation steps].

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How to set up and validate locally

1. Add a new metric to Usage Ping without adding its definition, for example: change line 77 in usage_data.rb to undefined_metric: 1, assignee_lists: count(List.assignee),
2. Run the specs edited in this MR - https://gitlab.com/gitlab-org/gitlab/blob/master/ee/spec/config/metrics/every_metric_definition_spec.rb
3. This spec should pass on master branch and fail on this MR's branch.

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