Fix small header filter bug. Add test

helper/metricsutil/metricsutil.go

@@ -45,7 +45,7 @@ func FormatFromRequest(req *logical.Request) string {
 
 		// Look for prometheus accept header
 		for _, header := range acceptHeaders {
-			if strings.Contains(header, PrometheusMetricFormat) {
+			if strings.Contains(header, PrometheusSchemaMIMEType) {
 				return PrometheusMetricFormat
 			}
 		}

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)
+		}
+	}
+}