Set OpenMetrics content header

See: https://github.com/prometheus/common/blob/main/expfmt/encode.go#L86

See: kubernetes#1973

pkg/metricshandler/metrics_handler.go

@@ -183,7 +183,7 @@ func (m *MetricsHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	resHeader := w.Header()
 	var writer io.Writer = w
 
-	resHeader.Set("Content-Type", `text/plain; version=`+"0.0.4")
+	resHeader.Set("Content-Type", `application/openmetrics-text; version=`+"0.0.1"+`; charset=utf-8`)
 
 	if m.enableGZIPEncoding {
 		// Gzip response if requested. Taken from
@@ -206,6 +206,8 @@ func (m *MetricsHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
+	w.Write([]byte("# EOF"))
+
 	// In case we gzipped the response, we have to close the writer.
 	if closer, ok := writer.(io.Closer); ok {
 		err := closer.Close()