Add tests for UsageMetricDefinitionGenerator generator

Using generator_spec gem for testing is not working properly, sometimes tests are failing, also this is not an active gem.

With this follow up-issue we could look for alternatives on testing the custom generators

The following discussion from !51201 (merged) should be addressed:

• @rspeicher started a discussion: (+2 comments)

  I'm wary of adding a gem for this, especially as this one seems to be inactive. Do we test any of our existing generators?

  Also I might expect this to be in the group above with rspec-rails. This group seems focused on test coverage.

  Should we add a version requirement?