Fix guidance on pushgateway authentication (#808)

The previous version of this example suggested to use the FastAPI
authentication helpers with @web_server. These do not work as the
pushgateway server is not a FastAPI server - we don't control its
authentication settings. This commit updates the documentation
appropriately.

10_integrations/pushgateway.py

@@ -46,8 +46,9 @@
 # conflict with each other.
 #
 # This is an example configuration, but a production-ready configuration will differ in two respects:
-# 1. You should set up authentication for the Pushgateway. Modal has built-in support
-#    for this with [our authentication helpers](https://modal.com/docs/guide/webhooks#authentication).
+# 1. You should set up authentication for the Pushgateway. Pushgateway has support for [basic authentication](https://github.com/prometheus/pushgateway/blob/42c4075fc5e2564031f2852885cdb2f5d570f672/README.md#tls-and-basic-authentication)
+#    out of the box. If you need more advanced authentication, consider using a [web endpoint with authentication](https://modal.com/docs/guide/webhooks#authentication)
+#    which proxies requests to the Pushgateway.
 # 2. The Pushgateway should listen on a [custom domain](https://modal.com/docs/guide/webhook-urls#custom-domains).
 #    This will allow you to configure Prometheus to scrape metrics from a predictable URL rather than
 #    the autogenerated URL Modal assigns to your app.