Metrics tween: ignore requests without a route

I'm seeing some errors come through now when requests for unhandled urls
(things like /ads.txt) are made, usually just by various bots/scanners.
The metrics tween is crashing on these, so this should fix it.

tildes/tildes/__init__.py

@@ -120,6 +120,10 @@ def metrics_tween_factory(handler: Callable, registry: Registry) -> Callable:
         response = handler(request)
         duration = time() - start_time
 
+        # ignore requests for invalid addresses
+        if not request.matched_route:
+            return response
+
         request_histogram.labels(
             route=request.matched_route.name,
             status_code=response.status_code,