Enable request queuing tracking in Datadog

Request queuing is critical to understand whether our app servers are
able to cope with all the traffic or requests pile up in Nginx queues.
I find the article
https://www.speedshop.co/2015/07/29/scaling-ruby-apps-to-1000-rpm.html
incredibly enlightening on this topic. Thanks @nateberkopec ❤️.

This requires
https://github.com/openfoodfoundation/ofn-install/pull/689.

config/initializers/datadog.rb

@@ -3,7 +3,9 @@ if ENV['DATADOG_RAILS_APM']
     c.use :rails, service_name: 'rails'
     c.use :delayed_job, service_name: 'delayed_job'
     c.use :dalli, service_name: 'memcached'
+
     c.analytics_enabled = true
     c.runtime_metrics_enabled = true
+    c.request_queuing = true
   end
 end