Fix incorrectly ordered arguments to the function and not properly merging lists.

lib/pleroma/web/metadata/providers/open_graph.ex

@@ -126,9 +126,10 @@ defp maybe_add_dimensions(metadata, url) do
     end
   end
 
-  defp maybe_add_video_thumbnail(url, metadata) do
+  defp maybe_add_video_thumbnail(metadata, url) do
     cond do
       Pleroma.Config.get([:media_preview_proxy, :enabled], false) ->
+        metadata ++
         [
           {:meta, [property: "og:image:width", content: "#{url["width"]}"], []},
           {:meta, [property: "og:image:height", content: "#{url["height"]}"], []},