Use the format_response hook for API requests

With endpoint_run, we were missing some Redis / DB calls that are
triggered during the serialization of data.

This happens very frequently because ActiveRecord relations are loaded
lazily and these are most likely going to be triggered during
serialization.

config/initializers/peek.rb

@@ -24,7 +24,7 @@ Peek.into Peek::Views::Tracing if Labkit::Tracing.tracing_url_enabled?
 # See https://github.com/peek/peek/blob/master/lib/peek/views/view.rb
 Peek.views
 
-ActiveSupport::Notifications.subscribe('endpoint_run.grape') do |_name, _start, _finish, _id, payload|
+ActiveSupport::Notifications.subscribe('format_response.grape') do |_name, _start, _finish, _id, payload|
   if request_id = payload[:env]['action_dispatch.request_id']
     Peek.adapter.save(request_id)
   end