allow numbers with underscore chars

Add support for numbers like 12_346_798 and 1.344_543_344

This is a convenience enhancement, which improves readability of long numbers in MetricsQL queries.
For example, 1_234_567_890 is easier to read than 1234567890.

These numbers are supported by Go - see golang/go#28493

lexer.go

@@ -233,7 +233,7 @@ func scanPositiveNumber(s string) (string, error) {
 		}
 		return s[:i], nil
 	}
-	for i < len(s) && isDecimalChar(s[i]) {
+	for i < len(s) && isDecimalCharOrUnderscore(s[i]) {
 		i++
 	}
 
@@ -258,7 +258,7 @@ func scanPositiveNumber(s string) (string, error) {
 		// Scan fractional part. It cannot be empty.
 		i++
 		j := i
-		for j < len(s) && isDecimalChar(s[j]) {
+		for j < len(s) && isDecimalCharOrUnderscore(s[j]) {
 			j++
 		}
 		i = j
@@ -673,6 +673,10 @@ func isDecimalChar(ch byte) bool {
 	return ch >= '0' && ch <= '9'
 }
 
+func isDecimalCharOrUnderscore(ch byte) bool {
+	return isDecimalChar(ch) || ch == '_'
+}
+
 func isHexChar(ch byte) bool {
 	return isDecimalChar(ch) || ch >= 'a' && ch <= 'f' || ch >= 'A' && ch <= 'F'
 }

lexer_test.go

@@ -75,6 +75,11 @@ func TestScanPositiveNumberSuccess(t *testing.T) {
 	f("2.3tb", "2.3tb")
 	f("3tI", "3tI")
 	f("4.5TIB   ", "4.5TIB")
+
+	// number with underscores - see https://github.com/golang/go/issues/28493
+	f("1_2_334", "1_2_334")
+	f("1_2.3_34_5", "1_2.3_34_5")
+	f("1_2.3_34_5e8", "1_2.3_34_5e8")
 }
 
 func TestScanPositiveNumberFailure(t *testing.T) {

parser_test.go

@@ -164,6 +164,8 @@ func TestParseSuccess(t *testing.T) {
 	// numberExpr
 	same(`1`)
 	same(`123.`)
+	same(`1_234`)
+	same(`1_2_34.56_78_9`)
 	another(`-123.`, `-123`)
 	same(`foo - 123.`)
 	same(`12.e+4`)