Enable trace & search analytics in Datadog

This will enrich the quality of the reported traces for all web
frameworks in our stack. That means not only memcached as we did in
https://github.com/openfoodfoundation/openfoodnetwork/pull/4266 but
PostgreSQL and Rails too.

After enabling, the Trace Search & Analytics page populates which allows
us to search traces and add APM queries to dashboards.

config/initializers/datadog.rb

@@ -2,5 +2,6 @@ if ENV['DATADOG_RAILS_APM']
   Datadog.configure do |c|
     c.use :rails, service_name: 'rails'
     c.use :delayed_job, service_name: 'delayed_job'
+    c.analytics_enabled = true
   end
 end