fix: Set Content-Type header for metrics endpoint (#808)

* fix: Set Content-Type header for metrics endpoint

* refactor

---------

Co-authored-by: Giovanni Barillari <giovanni.barillari@sentry.io>

Author: IngmarStein

src/metrics.rs

@@ -382,8 +382,10 @@ fn spawn_exporter(
                                 _ = connb.serve_connection(io, hyper::service::service_fn(move |_req| {
                                     let data = exp.get().data();
                                     async move {
+                                        let mut response = hyper::Response::new(http_body_util::Full::new(hyper::body::Bytes::from(data)));
+                                        response.headers_mut().append(hyper::header::CONTENT_TYPE, hyper::header::HeaderValue::from_static("text/plain"));
                                         Ok::<hyper::Response<http_body_util::Full<hyper::body::Bytes>>, std::convert::Infallible>(
-                                            hyper::Response::new(http_body_util::Full::new(hyper::body::Bytes::from(data)))
+                                            response
                                         )
                                     }
                                 })).await;