Fix prometheus registry (#613)

* Use registry passed to `setup_prometheus_sensors` to generate metric view

* refactor: import `web` once

faust/sensors/prometheus.py

@@ -5,7 +5,7 @@
 
 from aiohttp.web import Response
 
-from faust import web, web as _web
+from faust import web
 from faust.exceptions import ImproperlyConfigured
 from faust.types import (
     TP,
@@ -68,12 +68,12 @@ def setup_prometheus_sensors(
     app.monitor = PrometheusMonitor(metrics=faust_metrics)
 
     @app.page(pattern)
-    async def metrics_handler(self: _web.View, request: _web.Request) -> _web.Response:
+    async def metrics_handler(self: web.View, request: web.Request) -> web.Response:
         headers = {"Content-Type": CONTENT_TYPE_LATEST}
 
         return cast(
-            _web.Response,
-            Response(body=generate_latest(REGISTRY), headers=headers, status=200),
+            web.Response,
+            Response(body=generate_latest(registry), headers=headers, status=200),
         )