prefix with _ if name begins with a number

pkg/promutil/prometheus.go

@@ -13,6 +13,7 @@
 package promutil
 
 import (
+	"regexp"
 	"strings"
 	"time"
 
@@ -105,6 +106,8 @@ var replacer = strings.NewReplacer(
 	"%", "_percent",
 )
 
+var metricNumberPrefixRE = regexp.MustCompile(`^\d`)
+
 type PrometheusMetric struct {
 	Name             string
 	Labels           map[string]string
@@ -194,6 +197,11 @@ func PromStringTag(text string, labelsSnakeCase bool) (bool, string) {
 
 // sanitize replaces some invalid chars with an underscore
 func sanitize(text string) string {
+	// metrics starting with a digit violate the prometheus metric naming convention, so we add an underscore
+	if metricNumberPrefixRE.MatchString(text) {
+		text = "_" + text
+	}
+
 	if strings.ContainsAny(text, "“%") {
 		// fallback to the replacer for complex cases:
 		// - '“' is non-ascii rune

pkg/promutil/prometheus_test.go

@@ -63,6 +63,10 @@ func TestSanitize(t *testing.T) {
 			input:  "IHaveA%Sign",
 			output: "IHaveA_percentSign",
 		},
+		{
+			input:  "5XXError",
+			output: "_5XXError",
+		},
 	}
 
 	for _, tc := range testCases {