internal_event_matcher fail if no metric is defined

Problem

You should be able to create an event (foo) without a metric and then use internal_events_matchers in your specs like this:

  context 'foo event' do
    it 'triggers foo' do
      expect { fetch }.to trigger_internal_events('foo')
    end

    it 'does not trigger foo' do
      expect { nil }.not_to trigger_internal_events('foo')
    end
  end

However, the above will fail with:

1) Gitlab::LicenseScanning::PackageLicenses foo event triggers foo
     Failure/Error: raise ArgumentError, "Unknown event '#{event_name}'! #{name} matcher accepts only existing events"

     ArgumentError:
       Unknown event 'foo'! trigger_internal_events matcher accepts only existing events
[snip]

And

 2) Gitlab::LicenseScanning::PackageLicenses foo event does not trigger foo
     Failure/Error: raise ArgumentError, "Unknown event '#{event_name}'! #{name} matcher accepts only existing events"

     ArgumentError:
       Unknown event 'foo'! trigger_internal_events matcher accepts only existing events

Desired Outcome

You can use the matchers for testing events that are not used in metrics.

Potential Solution(s)

How to verify

Create an event foo and a spec with the above examples and see it pass.

Further actions needed