Use tags to mark individual middleware (github#17531)

lib/instrument-middleware.js

@@ -10,10 +10,10 @@ module.exports = function instrumentMiddleware (relativePath) {
   // Check if the middleware is an async function, to use the appropriate timer
   const isAsyncFunction = middleware.constructor.name === 'AsyncFunction'
 
-  // Name it `middleware.<filename>`
-  const name = `middleware.${path.basename(relativePath)}`
+  // Add a tag so we can see all middleware together
+  const tags = { middleware: path.basename(relativePath) }
 
   return isAsyncFunction
-    ? statsd.asyncTimer(middleware, name)
-    : statsd.timer(middleware, name)
+    ? statsd.asyncTimer(middleware, 'middleware', tags)
+    : statsd.timer(middleware, 'middleware', tags)
 }