The current Facet registration for plugins occurs independently of registering and using the plugin interface. To me, this breaks from the idea of centralizing additions plugins need in the plugin API. Current registration:

  Facets.register(Katello::Host::ContentFacet, :content_facet) do
    api_view :list => 'katello/api/v2/content_facet/base_with_root', :single => 'katello/api/v2/content_facet/show'
  end