feat: handle double dashes in unmapped metrics (#389)

Signed-off-by: akartasov <agneum@gmail.com>

pkg/mapper/escape.go

@@ -39,6 +39,7 @@ func EscapeMetricName(metricName string) string {
 	// This is an character replacement method optimized for this limited
 	// use case.  It is much faster than using a regex.
 	offset := 0
+	var prevChar rune
 	for i, c := range metricName {
 		// Seek forward, skipping valid characters until we find one that needs
 		// to be replaced, then add all the characters we've seen so far to the
@@ -47,6 +48,13 @@ func EscapeMetricName(metricName string) string {
 			(c >= '0' && c <= '9') || (c == '_') {
 			// Character is valid, so skip over it without doing anything.
 		} else {
+			// Double-dashes are allowed if there is a corresponding mapping.
+			// For consistency, double-dashes should also be allowed in the default case.
+			if c == '-' && prevChar == '-' {
+				offset = i + utf8.RuneLen(c)
+				continue
+			}
+
 			if !escaped {
 				// Up until now we've been lazy and avoided actually allocating
 				// memory.  Unfortunately we've now determined this string needs
@@ -58,6 +66,8 @@ func EscapeMetricName(metricName string) string {
 			offset = i + utf8.RuneLen(c)
 			sb.WriteByte('_')
 		}
+
+		prevChar = c
 	}
 
 	if !escaped {

pkg/mapper/escape_test.go

@@ -20,8 +20,10 @@ func TestEscapeMetricName(t *testing.T) {
 		"clean":                   "clean",
 		"0starts_with_digit":      "_0starts_with_digit",
 		"with_underscore":         "with_underscore",
+		"with--doubledash":        "with_doubledash",
+		"with---multiple-dashes":  "with_multiple_dashes",
 		"with.dot":                "with_dot",
-		"with😱emoji":              "with_emoji",
+		"with😱emoji":             "with_emoji",
 		"with.*.multiple":         "with___multiple",
 		"test.web-server.foo.bar": "test_web_server_foo_bar",
 		"":                        "",
@@ -39,6 +41,8 @@ func BenchmarkEscapeMetricName(b *testing.B) {
 		"clean",
 		"0starts_with_digit",
 		"with_underscore",
+		"with--doubledash",
+		"with---multiple-dashes",
 		"with.dot",
 		"with😱emoji",
 		"with.*.multiple",