fix metrics route normalization

src/utils/metrics.js

@@ -29,11 +29,12 @@ const httpRequestsInProgress = new client.Gauge({
   registers: [register]
 });
 
-function normalizeRoute(req) {
+function normalizeRoute(req, statusCode) {
   var routePath = req.route && req.route.path;
   var baseUrl = req.baseUrl || '';
   if (Array.isArray(routePath)) return baseUrl + routePath[0];
   if (routePath) return baseUrl + routePath;
+  if (statusCode === 404) return 'unmatched';
 
   var path = req.path || req.url || 'unknown';
   return path
@@ -45,13 +46,13 @@ function normalizeRoute(req) {
 function metricsMiddleware(req, res, next) {
   if (req.path === '/metrics') return next();
 
-  var route = normalizeRoute(req);
+  var route = req.route ? normalizeRoute(req) : 'pending';
   var method = req.method;
   var endTimer = httpRequestDuration.startTimer({ method, route });
   httpRequestsInProgress.inc({ method, route });
 
   res.on('finish', function() {
-    var finalRoute = normalizeRoute(req);
+    var finalRoute = normalizeRoute(req, res.statusCode);
     var labels = {
       method,
       route: finalRoute,

test/metrics.test.js

@@ -43,6 +43,25 @@ test('metrics middleware uses the first path for Express array routes', async ()
   assert.doesNotMatch(metrics, /route="\/,\/index\.html"/);
 });
 
+test('metrics middleware collapses unmatched 404 routes', async () => {
+  const req = {
+    method: 'GET',
+    path: '/wp-content/plugins/example.php',
+    url: '/wp-content/plugins/example.php?probe=1',
+    baseUrl: '',
+    route: null
+  };
+  const res = new EventEmitter();
+  res.statusCode = 404;
+
+  await new Promise((resolve) => metricsMiddleware(req, res, resolve));
+  res.emit('finish');
+
+  const metrics = await register.metrics();
+  assert.match(metrics, /route="unmatched"/);
+  assert.doesNotMatch(metrics, /wp-content\/plugins\/example\.php/);
+});
+
 test('metrics handler exposes prometheus content type', async () => {
   const headers = {};
   const res = {