expfmt: fix OpenMetrics 2.0 decoder error, format docs, and encoder version dispatch (#968)

* expfmt: fix OpenMetrics 2.0 decoder error, format docs, and encoder version dispatch

- Return unsupported format error in NewDecoder for OpenMetrics 2.0.
- Clarify experimental encode-only status and ignored EncoderOptions in doc comments.
- Use mime.ParseMediaType in NewEncoder for clean version dispatch.

Signed-off-by: David Ashpole <dashpole@google.com>

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Signed-off-by: David Ashpole <dashpole@google.com>

* Address Copilot review feedback: update NewDecoder and OpenMetrics 2.0 doc comments

Signed-off-by: David Ashpole <dashpole@google.com>

---------

Signed-off-by: David Ashpole <dashpole@google.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: dashpole

expfmt/decode.go

@@ -72,13 +72,15 @@ func ResponseFormat(h http.Header) Format {
 
 // NewDecoder returns a new decoder based on the given input format. Metric
 // names are validated based on the provided Format -- if the format requires
-// escaping, raditional Prometheues validity checking is used. Otherwise, names
+// escaping, traditional Prometheus validity checking is used. Otherwise, names
 // are checked for UTF-8 validity. Supported formats include delimited protobuf
-// and Prometheus text format.  For historical reasons, this decoder fallbacks
-// to classic text decoding for any other format. This decoder does not fully
-// support OpenMetrics although it may often succeed due to the similarities
-// between the formats. This decoder may not support the latest features of
-// Prometheus text format and is not intended for high-performance applications.
+// and Prometheus text format. For historical reasons, this decoder falls back
+// to classic text decoding for other legacy formats, but returns an error for
+// unsupported formats such as OpenMetrics 2.0. This decoder does not fully
+// support OpenMetrics although it may often succeed for OpenMetrics 1.0 due to
+// the similarities between the formats. This decoder may not support the latest
+// features of Prometheus text format and is not intended for high-performance
+// applications.
 // See: https://github.com/prometheus/common/issues/812
 func NewDecoder(r io.Reader, format Format) Decoder {
 	scheme := model.LegacyValidation
@@ -90,6 +92,11 @@ func NewDecoder(r io.Reader, format Format) Decoder {
 		return &protoDecoder{r: bufio.NewReader(r), s: scheme}
 	case TypeProtoText, TypeProtoCompact:
 		return &errDecoder{err: fmt.Errorf("format %s not supported for decoding", format)}
+	case TypeOpenMetrics:
+		_, params, err := mime.ParseMediaType(string(format))
+		if err == nil && params["version"] == OpenMetricsVersion_2_0_0 {
+			return &errDecoder{err: fmt.Errorf("format %s not supported for decoding", format)}
+		}
 	}
 	return &textDecoder{r: r, s: scheme}
 }

expfmt/decode_test.go

@@ -17,6 +17,7 @@ import (
 	"bufio"
 	"bytes"
 	"errors"
+	"fmt"
 	"io"
 	"math"
 	"net/http"
@@ -577,3 +578,69 @@ func TestTextDecoderWithBufioReader(t *testing.T) {
 	}
 	require.Truef(t, decoded, "Metric foo not decoded")
 }
+
+func TestNewDecoder(t *testing.T) {
+	om20Format, err := NewOpenMetricsFormat(OpenMetricsVersion_2_0_0)
+	require.NoError(t, err)
+
+	tests := []struct {
+		name        string
+		format      Format
+		expectError bool
+	}{
+		{
+			name:        "Text format",
+			format:      FmtText,
+			expectError: false,
+		},
+		{
+			name:        "ProtoDelim format",
+			format:      FmtProtoDelim,
+			expectError: false,
+		},
+		{
+			name:        "ProtoText format",
+			format:      FmtProtoText,
+			expectError: true,
+		},
+		{
+			name:        "ProtoCompact format",
+			format:      FmtProtoCompact,
+			expectError: true,
+		},
+		{
+			name:        "OpenMetrics 0.0.1",
+			format:      FmtOpenMetrics_0_0_1,
+			expectError: false,
+		},
+		{
+			name:        "OpenMetrics 1.0.0",
+			format:      FmtOpenMetrics_1_0_0,
+			expectError: false,
+		},
+		{
+			name:        "OpenMetrics 2.0.0",
+			format:      om20Format,
+			expectError: true,
+		},
+		{
+			name:        "OpenMetrics 2.0.0 with escaping",
+			format:      om20Format.WithEscapingScheme(model.ValueEncodingEscaping),
+			expectError: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			dec := NewDecoder(strings.NewReader(""), tt.format)
+			var mf dto.MetricFamily
+			err := dec.Decode(&mf)
+			if tt.expectError {
+				require.Error(t, err)
+				require.Contains(t, err.Error(), fmt.Sprintf("format %s not supported for decoding", tt.format))
+			} else if err != nil {
+				require.ErrorIs(t, err, io.EOF)
+			}
+		})
+	}
+}

expfmt/encode.go

@@ -16,8 +16,8 @@ package expfmt
 import (
 	"fmt"
 	"io"
+	"mime"
 	"net/http"
-	"strings"
 
 	"github.com/munnerz/goautoneg"
 	dto "github.com/prometheus/client_model/go"
@@ -222,7 +222,8 @@ func NewEncoder(w io.Writer, format Format, options ...EncoderOption) Encoder {
 			close: func() error { return nil },
 		}
 	case TypeOpenMetrics:
-		if strings.Contains(string(format), "version="+OpenMetricsVersion_2_0_0) {
+		_, params, err := mime.ParseMediaType(string(format))
+		if err == nil && params["version"] == OpenMetricsVersion_2_0_0 {
 			return encoderCloser{
 				encode: func(v *dto.MetricFamily) error {
 					_, err := MetricFamilyToOpenMetrics20(w, model.EscapeMetricFamily(v, escapingScheme), options...)

expfmt/encode_test.go

@@ -16,12 +16,14 @@ package expfmt
 import (
 	"bytes"
 	"net/http"
+	"strings"
 	"testing"
 
 	dto "github.com/prometheus/client_model/go"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"google.golang.org/protobuf/proto"
+	"google.golang.org/protobuf/types/known/timestamppb"
 
 	"github.com/prometheus/common/model"
 )
@@ -614,3 +616,75 @@ func BenchmarkNegotiateAccept(b *testing.B) {
 		_ = NegotiateAccept(h, accepted...)
 	}
 }
+
+func TestNewEncoder_OpenMetricsVersionDispatch(t *testing.T) {
+	counterMetric := &dto.MetricFamily{
+		Name: proto.String("test_counter"),
+		Type: dto.MetricType_COUNTER.Enum(),
+		Metric: []*dto.Metric{
+			{
+				Counter: &dto.Counter{
+					Value: proto.Float64(42),
+					CreatedTimestamp: &timestamppb.Timestamp{
+						Seconds: 1234567890,
+						Nanos:   0,
+					},
+				},
+			},
+		},
+	}
+
+	tests := []struct {
+		name         string
+		format       Format
+		expectedLine string
+	}{
+		{
+			name:         "OpenMetrics 1.0.0",
+			format:       FmtOpenMetrics_1_0_0,
+			expectedLine: "# TYPE test_counter unknown\ntest_counter 42.0\n",
+		},
+		{
+			name:         "OpenMetrics 0.0.1",
+			format:       FmtOpenMetrics_0_0_1,
+			expectedLine: "# TYPE test_counter unknown\ntest_counter 42.0\n",
+		},
+		{
+			name:         "OpenMetrics 2.0.0 standard",
+			format:       fmtOpenMetrics_2_0_0,
+			expectedLine: "# TYPE test_counter counter\ntest_counter 42.0 st@1234567890\n",
+		},
+		{
+			name:         "OpenMetrics 2.0.0 reordered parameters",
+			format:       Format("application/openmetrics-text; charset=utf-8; version=2.0.0"),
+			expectedLine: "# TYPE test_counter counter\ntest_counter 42.0 st@1234567890\n",
+		},
+		{
+			name:         "OpenMetrics 2.0.0 quoted version parameter",
+			format:       Format(`application/openmetrics-text; version="2.0.0"; charset=utf-8`),
+			expectedLine: "# TYPE test_counter counter\ntest_counter 42.0 st@1234567890\n",
+		},
+		{
+			name:         "OpenMetrics 2.0.0 with escaping scheme",
+			format:       Format("application/openmetrics-text; version=2.0.0; charset=utf-8; escaping=values"),
+			expectedLine: "# TYPE test_counter counter\ntest_counter 42.0 st@1234567890\n",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var buf bytes.Buffer
+			enc := NewEncoder(&buf, tt.format)
+			err := enc.Encode(counterMetric)
+			require.NoError(t, err)
+			closer, ok := enc.(Closer)
+			require.True(t, ok)
+			err = closer.Close()
+			require.NoError(t, err)
+
+			output := buf.String()
+			require.Contains(t, output, tt.expectedLine)
+			require.True(t, strings.HasSuffix(output, "# EOF\n"))
+		})
+	}
+}

expfmt/expfmt.go

@@ -111,6 +111,9 @@ func NewFormat(t FormatType) Format {
 
 // NewOpenMetricsFormat generates a new OpenMetrics format matching the
 // specified version number.
+//
+// Note: OpenMetrics version 2.0.0 is experimental and encode-only (currently
+// supporting counter, gauge, and untyped metric types).
 func NewOpenMetricsFormat(version string) (Format, error) {
 	if version == OpenMetricsVersion_0_0_1 {
 		return FmtOpenMetrics_0_0_1, nil
@@ -119,6 +122,7 @@ func NewOpenMetricsFormat(version string) (Format, error) {
 		return FmtOpenMetrics_1_0_0, nil
 	}
 	if version == OpenMetricsVersion_2_0_0 {
+		// OpenMetrics 2.0.0 is experimental and encode-only (counter/gauge/untyped).
 		return fmtOpenMetrics_2_0_0, nil
 	}
 	return FmtUnknown, errors.New("unknown open metrics version string")

expfmt/openmetrics_2_0_create.go

@@ -30,10 +30,14 @@ import (
 // OpenMetrics text format version 2.0.0 and writes the resulting lines to 'out'.
 // It returns the number of bytes written and any error encountered.
 //
-// NOTE: This method implements OpenMetrics 2.0-rc.0 which is experimental.
+// NOTE: This method targets OpenMetrics 2.0.0 (currently aligned with 2.0-rc.0) which is experimental and
+// encode-only (currently supporting counter, gauge, and untyped metric types).
 // Breaking changes might happen in the future. This implementation is still a
 // work-in-progress, and does not yet support all features of the format.
+// EncoderOptions are accepted for signature compatibility with
+// MetricFamilyToOpenMetrics and are currently ignored.
 func MetricFamilyToOpenMetrics20(out io.Writer, in *dto.MetricFamily, options ...EncoderOption) (written int, err error) {
+	// Options are accepted for signature compatibility and ignored.
 	_ = options
 	name := in.GetName()
 	if name == "" {