fix: fallback to gauge for protofmt-based negotiations

Fallback to `gauge` metric type when the negotiated content-type is
`protofmt`-based, since Prometheus' protobuf machinery does not
recognize all OpenMetrics types (`info` and `statesets` in this
context).

Signed-off-by: Pranshu Srivastava <rexagod@gmail.com>

pkg/metrics_store/metrics_writer.go

@@ -19,6 +19,11 @@ package metricsstore
 import (
 	"fmt"
 	"io"
+	"strings"
+
+	"github.com/prometheus/common/expfmt"
+
+	"k8s.io/kube-state-metrics/v2/pkg/metric"
 )
 
 // MetricsWriterList represent a list of MetricsWriter
@@ -82,13 +87,22 @@ func (m MetricsWriter) WriteAll(w io.Writer) error {
 	return nil
 }
 
-// SanitizeHeaders removes duplicate headers from the given MetricsWriterList for the same family (generated through CRS).
-// These are expected to be consecutive since G** resolution generates groups of similar metrics with same headers before moving onto the next G** spec in the CRS configuration.
-func SanitizeHeaders(writers MetricsWriterList) MetricsWriterList {
+// SanitizeHeaders sanitizes the headers of the given MetricsWriterList.
+func SanitizeHeaders(contentType string, writers MetricsWriterList) MetricsWriterList {
 	var lastHeader string
 	for _, writer := range writers {
 		if len(writer.stores) > 0 {
 			for i, header := range writer.stores[0].headers {
+				// If the requested content type was proto-based, replace the type with "gauge", as "info" and "statesets" are not recognized by Prometheus' protobuf machinery.
+				if strings.HasPrefix(contentType, expfmt.ProtoType) &&
+					strings.HasPrefix(header, "# HELP") &&
+					(strings.HasSuffix(header, " "+string(metric.Info)) || strings.HasSuffix(header, " "+string(metric.StateSet))) {
+					typeStringWithoutTypePaddedIndex := strings.LastIndex(header, " ")
+					typeStringWithoutType := header[:typeStringWithoutTypePaddedIndex]
+					writer.stores[0].headers[i] = typeStringWithoutType + " " + string(metric.Gauge)
+				}
+				// Removes duplicate headers from the given MetricsWriterList for the same family (generated through CRS).
+				// These are expected to be consecutive since G** resolution generates groups of similar metrics with same headers before moving onto the next G** spec in the CRS configuration.
 				if header == lastHeader {
 					writer.stores[0].headers[i] = ""
 				} else {

pkg/metrics_store/metrics_writer_test.go

@@ -18,6 +18,7 @@ package metricsstore_test
 
 import (
 	"fmt"
+	"github.com/prometheus/common/expfmt"
 	"strings"
 	"testing"
 
@@ -263,3 +264,49 @@ func TestWriteAllWithEmptyStores(t *testing.T) {
 		t.Fatalf("Unexpected output, got %q, want %q", result, "")
 	}
 }
+
+func BenchmarkSanitizeHeaders(b *testing.B) {
+	benchmarks := []struct {
+		name                      string
+		contentType               expfmt.Format
+		writersContainsDuplicates bool
+	}{
+		{
+			name:                      "text-format unique headers",
+			contentType:               expfmt.FmtText,
+			writersContainsDuplicates: false,
+		},
+		{
+			name:                      "text-format duplicate headers",
+			contentType:               expfmt.FmtText,
+			writersContainsDuplicates: true,
+		},
+		{
+			name:                      "proto-format unique headers",
+			contentType:               expfmt.ProtoFmt, // Prometheus ProtoFmt is the only proto-based format we check for.
+			writersContainsDuplicates: false,
+		},
+		{
+			name:                      "proto-format duplicate headers",
+			contentType:               expfmt.ProtoFmt, // Prometheus ProtoFmt is the only proto-based format we check for.
+			writersContainsDuplicates: true,
+		},
+	}
+
+	for _, benchmark := range benchmarks {
+		headers := []string{}
+		for j := 0; j < 10e4; j++ {
+			if benchmark.writersContainsDuplicates {
+				headers = append(headers, fmt.Sprintf("# HELP foo foo_help\n# TYPE foo info"))
+			} else {
+				headers = append(headers, fmt.Sprintf("# HELP foo_%d foo_help\n# TYPE foo_%d info", j, j))
+			}
+		}
+		writer := metricsstore.NewMetricsWriter(metricsstore.NewMetricsStore(headers, nil))
+		b.Run(benchmark.name, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				metricsstore.SanitizeHeaders(string(benchmark.contentType), metricsstore.MetricsWriterList{writer})
+			}
+		})
+	}
+}

pkg/metricshandler/metrics_handler.go

@@ -187,6 +187,7 @@ func (m *MetricsHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	var writer io.Writer = w
 
 	contentType := expfmt.NegotiateIncludingOpenMetrics(r.Header)
+	gotContentType := contentType
 
 	// We do not support protobuf at the moment. Fall back to FmtText if the negotiated exposition format is not FmtOpenMetrics See: https://github.com/kubernetes/kube-state-metrics/issues/2022
 	if contentType != expfmt.FmtOpenMetrics_1_0_0 && contentType != expfmt.FmtOpenMetrics_0_0_1 {
@@ -208,7 +209,7 @@ func (m *MetricsHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
-	m.metricsWriters = metricsstore.SanitizeHeaders(m.metricsWriters)
+	m.metricsWriters = metricsstore.SanitizeHeaders(string(gotContentType), m.metricsWriters)
 	for _, w := range m.metricsWriters {
 		err := w.WriteAll(writer)
 		if err != nil {