docs: Add pointer to debugPrintReports for AER (#3092)

* docs: Add pointer to `debugPrintReports` for aer

This allows users to validate why metrics may not be working if they encounter errors.

* Apply suggestions from code review

Thanks Jesse!

Co-Authored-By: Jesse Rosenberger <git@jro.cc>

* Feedback on https://github.com/apollographql/apollo-server/pull/3092\#discussion_r316842126.

* Feedback for #3092 (comment).

docs/source/features/metrics.md

@@ -57,6 +57,22 @@ You can set environment variable values on the command line as seen below, or wi
 ENGINE_API_KEY=YOUR_API_KEY ENGINE_SCHEMA_TAG=development node start-server.js
 ```
 
+### Debugging Graph Manager reporting
+
+You can set the [`debugPrintReports` option](https://github.com/apollographql/apollo-server/blob/master/packages/apollo-engine-reporting/src/agent.ts#L429-L433) in the `engine` section of the `ApolloServer` constructor to automatically log debugging information for all reporting requests sent to Graph Manager.  For example:
+
+```js{8}
+const { ApolloServer } = require("apollo-server");
+
+const server = new ApolloServer({
+  typeDefs,
+  resolvers,
+  engine: {
+    debugPrintReports: true,
+  }
+});
+```
+
 ### Identifying distinct clients
 
 Graph Manager's [client awareness feature](https://www.apollographql.com/docs/platform/client-awareness) enables you to view metrics for distinct versions