nit, trailing whitespace added

nit, missing trailing newline

Should this field be on RouteBackend? so that backend errors can be raised on FirstAvailable distributions.

makes sense!

I'd be inclined to move this as a submessage of RouteBackend, where it's actually used, and perhaps call it something like Invalid, to make it clear that this is a failure used to indicate an illegal backend configuration.

Rather than use free-form string messages to indicate denials, I suspect we want a oneof or enum to clearly enumerate the types of route errors that may be emitted.

I'd also be inclined to move this message within the Route messages, as we do for other types.

@olix0r @adleong

Let me know if that works or we want to make it into something like:

  message RouteBackend {
    oneof kind {
      Backend valid = 1;
      Invalid invalid = 2;
    }
        
    message Invalid {
      string message = 1;
    }
  }