impl<S, B> Future for MetricsResponse<S, B>
where
    B: MessageBody,
    S: Service<ServiceRequest, Response = ServiceResponse<B>, Error = Error>,
{
    type Output = Result<ServiceResponse<StreamMetrics<B>>, Error>;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        let this = self.project();

        let start = *this.start;

        let res = match futures::ready!(this.fut.poll(cx)) {
            Ok(res) => res,
            Err(e) => return Poll::Ready(Err(e)),
        };

        let req = res.request();
        let method = req.method().clone();
        let pattern_or_path = req
            .match_pattern()
            .unwrap_or_else(|| req.path().to_string());
        let path = req.path().to_string();
        let inner = this.inner.clone();

        Poll::Ready(Ok(res.map_body(move |mut head, mut body| {
            // We short circuit the response status and body to serve the endpoint
            // automagically. This way the user does not need to set the middleware *AND*
            // an endpoint to serve middleware results. The user is only required to set
            // the middleware and tell us what the endpoint should be.
            if inner.matches(&path, &method) {
                head.status = StatusCode::OK;
                head.headers.insert(
                    CONTENT_TYPE,
                    HeaderValue::from_static("text/plain; version=0.0.4; charset=utf-8"),
                );
            }

            StreamMetrics {
                body: B::from(inner.metrics()),
                size: 0,
                start,
                end: inner.clock.end(),
                inner,
                status: head.status,
                path: pattern_or_path,
                method,
            }
        })))
    }
}

error[E0308]: mismatched types
   --> app/src/metrics.rs:176:31
    |
137 | impl<S, B> Future for MetricsResponse<S, B>
    |         - this type parameter
...
176 |                 body: B::from(inner.metrics()),
    |                               ^^^^^^^^^^^^^^^ expected type parameter `B`, found struct `std::string::String`
    |
    = note: expected type parameter `B`
                       found struct `std::string::String`