Extending Building a middleware from scratch guide

[samueljoli]

The Building a middleware from scratch is a great guide, I think it would be an improvement if the following steps were added:

• Creating a layer

impl TimeoutLayer {
    /// Create a timeout from a duration
    pub fn new(timeout: Duration) -> Self {
        TimeoutLayer { timeout }
    }
}

impl<S> Layer<S> for TimeoutLayer {
    type Service = Timeout<S>;

    fn layer(&self, service: S) -> Self::Service {
        Timeout::new(service, self.timeout)
    }
}

• Error handling for middleware

• Using the layer (integration with ServiceBuilder