[Draft] Add multiple unique properties handling

What does this MR do and why?

Related to #415139 (closed)

Script used to generate the legacy events file:

events = Gitlab::InternalEvents::EventDefinitions.send(:events)
events = events.map do |event, props|
  props = props.map do |prop|
    next prop if prop.nil?
    prop.to_s
  end
  [event, props]
end.to_h
path = Rails.root.join('config', 'metrics', 'legacy_events_properties.yml')
File.open(path, 'w') do |f|
  f.write(events.to_yaml)
end

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