feat(express): Add option of passing router to tracing integration (#2740)

As of getsentry/sentry-javascript#3078 the integration can also take a router instead of an app.

Author: lobsterkatie

src/platforms/node/guides/express/index.mdx

@@ -165,7 +165,12 @@ Sentry.init({
     // enable HTTP calls tracing
     new Sentry.Integrations.Http({ tracing: true }),
     // enable Express.js middleware tracing
-    new Tracing.Integrations.Express({ app }),
+    new Tracing.Integrations.Express({ 
+      // to trace all requests to the default router
+      app, 
+      // alternatively, you can specify the routes you want to trace:
+      // router: someRouter, 
+    }),
   ],
 
   // We recommend adjusting this value in production, or using tracesSampler