Merge pull request #4281 from Matt-Yorkley/memcached-apm

Add memcached APM

config/initializers/datadog.rb

@@ -2,6 +2,7 @@ if ENV['DATADOG_RAILS_APM']
   Datadog.configure do |c|
     c.use :rails, service_name: 'rails'
     c.use :delayed_job, service_name: 'delayed_job'
+    c.use :dalli, service_name: 'memcached'
     c.analytics_enabled = true
   end
 end