feat: Whitelists the gatherStats option. (#2561)

* feat: Whitelists the gatherStats option.

* doc: Replaces obsolete disableStats option with gatherStats.

config.js

     // Stats
     //
 
-    // Whether to enable stats collection or not.
-    // disableStats: false,
+    // Whether to enable stats collection or not in the TraceablePeerConnection.
+    // This can be useful for debugging purposes (post-processing/analysis of
+    // the webrtc stats) as it is done in the jitsi-meet-torture bandwidth
+    // estimation tests.
+    // gatherStats: false,
 
     // To enable sending statistics to callstats.io you must provide the
     // Application ID and Secret.

react/features/base/config/functions.js

     'failICE',
     'firefox_fake_device',
     'forceJVB121Ratio',
+    'gatherStats',
     'hiddenDomain',
     'hosts',
     'iAmRecorder',