[Prod] Change how Datadog is enabled

This works fine here (filled in through dockers .env injection)
but on prod the application doesn't know about the API key

.env.sample

@@ -71,6 +71,7 @@
 
 # Required:
 # DD_API_KEY=
+# DD_ENABLE=true
 #
 # Optional:
 # DD_SITE=us5.datadoghq.com

config/initializers/datadog.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 Datadog.configure do |c|
-  c.tracing.enabled = !Rails.env.test? && ENV["DD_API_KEY"].present?
+  c.tracing.enabled = !Rails.env.test? && ENV["DD_ENABLE"] == "true"
   c.logger.level = Logger::WARN
 
   c.tracing.instrument :rack, quantize: {