Track root render

Reviewed By: lblasa

Differential Revision: D50121104

fbshipit-source-id: 7f3560fe67f3a84e21fe7819a7f3a0b889716cc9

desktop/flipper-ui-core/src/startFlipperDesktop.tsx

@@ -123,6 +123,10 @@ class AppFrame extends React.Component<
     );
   }
 
+  componentDidMount(): void {
+    uiPerfTracker.track('ui-perf-root-rendered');
+  }
+
   componentDidCatch(error: any, errorInfo: any) {
     console.error(
       `Flipper chrome crash: ${error}`,

desktop/flipper-ui-core/src/utils/UIPerfTracker.tsx

@@ -11,6 +11,7 @@ import {getLogger} from 'flipper-common';
 
 type UIPerfEvents =
   | 'ui-perf-store-rehydrated'
+  | 'ui-perf-root-rendered'
   | 'ui-perf-everything-finally-loaded-jeeeez';
 
 class UIPerfTracker {