[Usage Collection] Add legacy=true option to the /api/stats request in the docs (#75146)

Author: afharo

src/plugins/usage_collection/README.md

@@ -177,7 +177,7 @@ New fields added to the telemetry payload currently mean that telemetry cluster
 
 There are a few ways you can test that your usage collector is working properly.
 
-1. The `/api/stats?extended=true` HTTP API in Kibana (added in 6.4.0) will call the fetch methods of all the registered collectors, and add them to a stats object you can see in a browser or in curl. To test that your usage collector has been registered correctly and that it has the model of data you expected it to have, call that HTTP API manually and you should see a key in the `usage` object of the response named after your usage collector's `type` field. This method tests the Metricbeat scenario described above where `callCluster` wraps `callWithRequest`.
+1. The `/api/stats?extended=true&legacy=true` HTTP API in Kibana (added in 6.4.0) will call the fetch methods of all the registered collectors, and add them to a stats object you can see in a browser or in curl. To test that your usage collector has been registered correctly and that it has the model of data you expected it to have, call that HTTP API manually and you should see a key in the `usage` object of the response named after your usage collector's `type` field. This method tests the Metricbeat scenario described above where `callCluster` wraps `callWithRequest`.
 2. There is a dev script in x-pack that will give a sample of a payload of data that gets sent up to the telemetry cluster for the sending phase of telemetry. Collected data comes from:
     - The `.monitoring-*` indices, when Monitoring is enabled. Monitoring enhances the sent payload of telemetry by producing usage data potentially of multiple clusters that exist in the monitoring data. Monitoring data is time-based, and the time frame of collection is the last 15 minutes.
     - Live-pulled from ES API endpoints. This will get just real-time stats without context of historical data.