Sanitize metric names in the prometheus exporter

CHANGELOG.md

@@ -19,6 +19,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - The metric SDK in `go.opentelemetry.io/otel/sdk/metric` is completely refactored to comply with the OpenTelemetry specification.
   Please see the package documentation for how the new SDK is initialized and configured. (#3175)
 - Update the minimum supported go version to go1.18. Removes support for go1.17 (#3179)
+- Instead of dropping metric, the Prometheus exporter will replace any invalid character in metric names with `_`. (#3212)
 
 ### Removed
 

exporters/prometheus/exporter.go

@@ -145,7 +145,7 @@ func getHistogramMetricData(histogram metricdata.Histogram, m metricdata.Metrics
 	dataPoints := make([]*metricData, 0, len(histogram.DataPoints))
 	for _, dp := range histogram.DataPoints {
 		keys, values := getAttrs(dp.Attributes)
-		desc := prometheus.NewDesc(m.Name, m.Description, keys, nil)
+		desc := prometheus.NewDesc(sanitizeName(m.Name), m.Description, keys, nil)
 		buckets := make(map[float64]uint64, len(dp.Bounds))
 		for i, bound := range dp.Bounds {
 			buckets[bound] = dp.BucketCounts[i]
@@ -168,7 +168,7 @@ func getSumMetricData[N int64 | float64](sum metricdata.Sum[N], m metricdata.Met
 	dataPoints := make([]*metricData, 0, len(sum.DataPoints))
 	for _, dp := range sum.DataPoints {
 		keys, values := getAttrs(dp.Attributes)
-		desc := prometheus.NewDesc(m.Name, m.Description, keys, nil)
+		desc := prometheus.NewDesc(sanitizeName(m.Name), m.Description, keys, nil)
 		md := &metricData{
 			name:            m.Name,
 			description:     desc,
@@ -185,7 +185,7 @@ func getGaugeMetricData[N int64 | float64](gauge metricdata.Gauge[N], m metricda
 	dataPoints := make([]*metricData, 0, len(gauge.DataPoints))
 	for _, dp := range gauge.DataPoints {
 		keys, values := getAttrs(dp.Attributes)
-		desc := prometheus.NewDesc(m.Name, m.Description, keys, nil)
+		desc := prometheus.NewDesc(sanitizeName(m.Name), m.Description, keys, nil)
 		md := &metricData{
 			name:            m.Name,
 			description:     desc,
@@ -233,3 +233,51 @@ func sanitizeRune(r rune) rune {
 	}
 	return '_'
 }
+
+func sanitizeName(n string) string {
+	// This algorithm is based on strings.Map from Go 1.19.
+	const (
+		replacement = '_'
+		width       = 1
+	)
+
+	valid := func(i int, r rune) bool {
+		// Taken from
+		// https://github.com/prometheus/common/blob/dfbc25bd00225c70aca0d94c3c4bb7744f28ace0/model/metric.go#L92-L102
+		if (r >= 'a' && r <= 'z') || (r >= 'A' && r <= 'Z') || r == '_' || r == ':' || (r >= '0' && r <= '9' && i > 0) {
+			return true
+		}
+		return false
+	}
+
+	// This output buffer b is initialized on demand, the first time a character
+	// needs to be replaced.
+	var b strings.Builder
+	for i, c := range n {
+		if valid(i, c) {
+			continue
+		}
+		b.Grow(len(n))
+		b.WriteString(n[:i])
+		b.WriteRune(replacement)
+		n = n[i+width:]
+		break
+	}
+
+	// Fast path for unchanged input.
+	if b.Cap() == 0 { // b.Grow was not called above.
+		return n
+	}
+
+	for _, c := range n {
+		// Due to inlining, it is more performant to invoke WriteByte rather then
+		// WriteRune.
+		if valid(1, c) { // We are guaranteed to not be at the start.
+			b.WriteByte(byte(c))
+		} else {
+			b.WriteByte(byte(replacement))
+		}
+	}
+
+	return b.String()
+}

exporters/prometheus/exporter_test.go

@@ -106,8 +106,8 @@ func TestPrometheusExporter(t *testing.T) {
 			},
 		},
 		{
-			name:         "invalid instruments are dropped",
-			expectedFile: "testdata/gauge.txt",
+			name:         "invalid instruments are renamed",
+			expectedFile: "testdata/sanitized_names.txt",
 			recordMetrics: func(ctx context.Context, meter otelmetric.Meter) {
 				attrs := []attribute.KeyValue{
 					attribute.Key("A").String("B"),
@@ -119,16 +119,16 @@ func TestPrometheusExporter(t *testing.T) {
 				gauge.Add(ctx, 100, attrs...)
 				gauge.Add(ctx, -25, attrs...)
 
-				// Invalid, should be dropped.
-				gauge, err = meter.SyncFloat64().UpDownCounter("invalid.gauge.name")
+				// Invalid, will be renamed.
+				gauge, err = meter.SyncFloat64().UpDownCounter("invalid.gauge.name", instrument.WithDescription("a gauge with an invalid name"))
 				require.NoError(t, err)
 				gauge.Add(ctx, 100, attrs...)
 
-				counter, err := meter.SyncFloat64().Counter("invalid.counter.name")
+				counter, err := meter.SyncFloat64().Counter("0invalid.counter.name", instrument.WithDescription("a counter with an invalid name"))
 				require.NoError(t, err)
 				counter.Add(ctx, 100, attrs...)
 
-				histogram, err := meter.SyncFloat64().Histogram("invalid.hist.name")
+				histogram, err := meter.SyncFloat64().Histogram("invalid.hist.name", instrument.WithDescription("a histogram with an invalid name"))
 				require.NoError(t, err)
 				histogram.Record(ctx, 23, attrs...)
 			},

exporters/prometheus/testdata/sanitized_names.txt

@@ -0,0 +1,24 @@
+# HELP bar a fun little gauge
+# TYPE bar counter
+bar{A="B",C="D"} 75
+# HELP _invalid_counter_name a counter with an invalid name
+# TYPE _invalid_counter_name counter
+_invalid_counter_name{A="B",C="D"} 100
+# HELP invalid_gauge_name a gauge with an invalid name
+# TYPE invalid_gauge_name counter
+invalid_gauge_name{A="B",C="D"} 100
+# HELP invalid_hist_name a histogram with an invalid name
+# TYPE invalid_hist_name histogram
+invalid_hist_name_bucket{A="B",C="D",le="0"} 0
+invalid_hist_name_bucket{A="B",C="D",le="5"} 0
+invalid_hist_name_bucket{A="B",C="D",le="10"} 0
+invalid_hist_name_bucket{A="B",C="D",le="25"} 1
+invalid_hist_name_bucket{A="B",C="D",le="50"} 0
+invalid_hist_name_bucket{A="B",C="D",le="75"} 0
+invalid_hist_name_bucket{A="B",C="D",le="100"} 0
+invalid_hist_name_bucket{A="B",C="D",le="250"} 0
+invalid_hist_name_bucket{A="B",C="D",le="500"} 0
+invalid_hist_name_bucket{A="B",C="D",le="1000"} 0
+invalid_hist_name_bucket{A="B",C="D",le="+Inf"} 1
+invalid_hist_name_sum{A="B",C="D"} 23
+invalid_hist_name_count{A="B",C="D"} 1