fix: Use Prometheus standard content-type (#1022)

Signed-off-by: Carlos Juan Gómez Peñalver <[email protected]>

Author: carlosjgp

serve.go

@@ -186,5 +186,9 @@ func (h healthHandler) responseContentType(outputName string) string {
 	if outputName == "json" {
 		return "application/json"
 	}
+	if outputName == "prometheus" {
+		return "text/plain; version=0.0.4"
+	}
+
 	return fmt.Sprintf("%s%s", mediaTypePrefix, outputName)
 }

serve_test.go

@@ -117,6 +117,15 @@ func TestServeNegotiatingContent(t *testing.T) {
 			expectedHTTPStatus:  http.StatusOK,
 			expectedContentType: "application/json",
 		},
+		"accept prometheus": {
+			acceptHeader: []string{
+				"text/plain; version=0.0.4",
+			},
+			outputFormat:        "prometheus",
+			specFile:            filepath.Join("testdata", "passing.goss.yaml"),
+			expectedHTTPStatus:  http.StatusOK,
+			expectedContentType: "text/plain; version=0.0.4",
+		},
 		"accept header contains vendor-specific output format different from process-level": {
 			acceptHeader: []string{
 				"application/vnd.goss-rspecish",