Track proxy errors

Summary:
Need to answer:
- Proxy errors rate over time

Reviewed By: antonk52

Differential Revision: D46219927

fbshipit-source-id: 1ad8cc60303ed3aa34828b9ff87009ff7754a146

desktop/flipper-server-core/src/server/startServer.tsx

@@ -212,8 +212,7 @@ async function startProxyServer(
       res.writeHead(502, 'Failed to proxy request');
     }
     res.end('Failed to proxy request: ' + err);
-    // TODO: should exit as proxying requests will continue to fail.
-    // TODO: track these instances.
+    tracker.track('server-proxy-error', {error: err.message});
   });
 
   return new Promise((resolve) => {

desktop/flipper-server-core/src/utils/tracker.tsx

@@ -16,6 +16,7 @@ type TrackerEvents = {
     error?: string;
   };
   'server-socket-already-in-use': {};
+  'server-proxy-error': {error: string};
 };
 
 class ServerCoreTracker {