Add accept header check for prometheus mime type (#7958)

* Add accept header check for prometheus mime type

* Fix small header filter bug. Add test

helper/metricsutil/metricsutil.go

@@ -16,6 +16,8 @@ import (
 const (
 	OpenMetricsMIMEType = "application/openmetrics-text"
 
+	PrometheusSchemaMIMEType = "prometheus/telemetry"
+
 	// ErrorContentType is the content type returned by an error response.
 	ErrorContentType = "text/plain"
 )
@@ -38,7 +40,14 @@ func FormatFromRequest(req *logical.Request) string {
 	if len(acceptHeaders) > 0 {
 		acceptHeader := acceptHeaders[0]
 		if strings.HasPrefix(acceptHeader, OpenMetricsMIMEType) {
-			return "prometheus"
+			return PrometheusMetricFormat
+		}
+
+		// Look for prometheus accept header
+		for _, header := range acceptHeaders {
+			if strings.Contains(header, PrometheusSchemaMIMEType) {
+				return PrometheusMetricFormat
+			}
 		}
 	}
 	return ""

helper/metricsutil/metricsutil_test.go

@@ -0,0 +1,45 @@
+package metricsutil
+
+import (
+	"github.com/hashicorp/vault/sdk/logical"
+	"testing"
+)
+
+func TestFormatFromRequest(t *testing.T) {
+	testCases := []struct {
+		original *logical.Request
+		expected string
+	}{
+		{
+			original: &logical.Request{Headers: map[string][]string{
+				"Accept": {
+					"application/vnd.google.protobuf",
+					"schema=\"prometheus/telemetry\"",
+				},
+			}},
+			expected: "prometheus",
+		},
+		{
+			original: &logical.Request{Headers: map[string][]string{
+				"Accept": {
+					"schema=\"prometheus\"",
+				},
+			}},
+			expected: "",
+		},
+		{
+			original: &logical.Request{Headers: map[string][]string{
+				"Accept": {
+					"application/openmetrics-text",
+				},
+			}},
+			expected: "prometheus",
+		},
+	}
+
+	for _, tCase := range testCases {
+		if metricsType := FormatFromRequest(tCase.original); metricsType != tCase.expected {
+			t.Fatalf("expected %s but got %s", tCase.expected, metricsType)
+		}
+	}
+}