fix(screenshare) Add timestamp to desktop track

Send screenshare duration to analytics

conference.js

     getLocalJitsiAudioTrack,
     getLocalJitsiVideoTrack,
     getLocalTracks,
+    getLocalVideoTrack,
     isLocalCameraTrackMuted,
     isLocalTrackMuted,
     isUserInteractionRequiredForUnmute,
         if (config.enableScreenshotCapture) {
             APP.store.dispatch(toggleScreenshotCaptureSummary(false));
         }
+        const tracks = APP.store.getState()['features/base/tracks'];
+        const timestamp = getLocalVideoTrack(tracks)?.timestamp ?? 0;
 
         // It can happen that presenter GUM is in progress while screensharing is being turned off. Here it needs to
         // wait for that GUM to be resolved in order to prevent leaking the presenter track(this.localPresenterVideo
         return promise.then(
             () => {
                 this.videoSwitchInProgress = false;
-                sendAnalytics(createScreenSharingEvent('stopped'));
+                sendAnalytics(createScreenSharingEvent('stopped',
+                    timestamp === 0 ? null : (Date.now() / 1000) - timestamp));
                 logger.info('Screen sharing stopped.');
             },
             error => {

react/features/analytics/AnalyticsEvents.js

  * occurred (e.g. It was started or stopped).
  *
  * @param {string} action - The action which occurred.
+ * @param {number?} value - The screenshare duration in seconds.
  * @returns {Object} The event in a format suitable for sending via
  * sendAnalytics.
  */
-export function createScreenSharingEvent(action) {
+export function createScreenSharingEvent(action, value = null) {
     return {
         action,
-        actionSubject: 'screen.sharing'
+        actionSubject: 'screen.sharing',
+        attributes: {
+            value
+        }
     };
 }
 

react/features/base/tracks/middleware.js

         // The devices list needs to be refreshed when no initial video permissions
         // were granted and a local video track is added by umuting the video.
         if (action.track.local) {
+            action.track.timestamp = Date.now() / 1000;
             store.dispatch(getAvailableDevices());
         }