Add definition for WAU and MAU to Service Ping page

doc/development/service_ping/index.md

@@ -50,6 +50,8 @@ We use the following terminology to describe the Service Ping components:
 - **Metrics**: primarily made up of row counts for different tables in an instance's database. Each
   metric has a corresponding [metric definition](metrics_dictionary.md#metrics-definition-and-validation)
   in a YAML file.
+- **MAU**: monthly active users.
+- **WAU**: weekly active users. 
 
 ### Why should we enable Service Ping?