Instrument preview image drawing

activestorage/lib/active_storage/previewer.rb

@@ -43,9 +43,11 @@ def preview
       #
       # The output tempfile is opened in the directory returned by ActiveStorage::Downloading#tempdir.
       def draw(*argv) #:doc:
-        Tempfile.open("ActiveStorage", tempdir) do |file|
-          capture(*argv, to: file)
-          yield file
+        ActiveSupport::Notifications.instrument("preview.active_storage") do
+          Tempfile.open("ActiveStorage", tempdir) do |file|
+            capture(*argv, to: file)
+            yield file
+          end
         end
       end