text: Title: SentryMiddleware
  Body: I've taken a first pass at a [Sentry integration for ASGI](https://github.com/encode/sentry-asgi).
  
  Things you probably want to do to support it well:
  
  * Document `.add_middleware` for adding ASGI middleware. (Perhaps with a section linking to third party ASGI middleware implementations?)
  * Ensure the router updates the ASGI scope with an 'endpoint', which should be the routed class or function.