stats: Add metrics for all HTTP status codes

src/node/hooks/express.js

@@ -1,6 +1,7 @@
 'use strict';
 
 const _ = require('underscore');
+const assert = require('assert').strict;
 const cookieParser = require('cookie-parser');
 const events = require('events');
 const express = require('express');
@@ -112,14 +113,14 @@ exports.restartServer = async () => {
     exports.server = http.createServer(app);
   }
 
-  // This error-handling middleware is installed first to ensure that no other middleware can
+  app.use((req, res, next) => {
+    // This stats logic should be in the first middleware to ensure that no other middleware can
     // prevent the stats from being updated.
-  app.use((err, req, res, next) => {
-    stats.meter('http500').mark();
-    next(err);
+    res.on('finish', () => {
+      assert(res.statusCode);
+      stats.meter(`http${res.statusCode}`).mark();
     });
 
-  app.use((req, res, next) => {
     // res.header("X-Frame-Options", "deny"); // breaks embedded pads
     if (settings.ssl) {
       // we use SSL