Use existing environment variables for cloudwatch

We already have DD_SERVICE and DD_ENV that we use for datadog,
these might as well also be used for the request queue time metric.

README.md

@@ -21,8 +21,14 @@ And then execute:
 The following environment variables are required for the metric that is reported to cloudwatch:
 
 ```rb
-    ENV["APP_NAME"]
-    ENV["SERVER_ENVIRONMENT"]
+    ENV["DD_SERVICE"]
+    ENV["DD_ENV"]
+```
+
+For example:
+```rb
+    ENV["DD_SERVICE"] = "teamtailor"
+    ENV["DD_ENV"] = "eu-cyan"
 ```
 
 For the Reporter to run, you need to have the following flag set as well:

lib/services/auto_scaling_metrics/reporter.rb

@@ -12,8 +12,8 @@ class Reporter
       include Singleton
 
       DIMENSIONS = [
-        {name: "service", value: ENV["APP_NAME"]},
-        {name: "environment", value: ENV["SERVER_ENVIRONMENT"]}
+        {name: "service", value: ENV["DD_SERVICE"]},
+        {name: "environment", value: ENV["DD_ENV"]}
       ]
 
       attr_accessor :collector