Add preload attribute to markdown videos

changelogs/unreleased/321297-load-only-metadata-for-markdown-video-include.yml

+---
+title: Add preload attribute to markdown videos
+merge_request: 54350
+author:
+type: performance

lib/banzai/filter/video_link_filter.rb

@@ -15,7 +15,7 @@ module Banzai
       end
 
       def extra_element_attrs
-        { width: "400" }
+        { width: "400", preload: "metadata" }
       end
     end
   end

spec/lib/banzai/filter/video_link_filter_spec.rb

@@ -33,6 +33,7 @@ RSpec.describe Banzai::Filter::VideoLinkFilter do
       expect(video.name).to eq 'video'
       expect(video['src']).to eq src
       expect(video['width']).to eq "400"
+      expect(video['preload']).to eq 'metadata'
 
       expect(paragraph.name).to eq 'p'