Prevent lexer from seeking to next rune after lexing escape sequence. (#8517)

* Prevent lexer from seeking to next rune after lexing escape sequence.

Signed-off-by: Danny Kopping <danny.kopping@grafana.com>

promql/parser/lex.go

@@ -583,8 +583,12 @@ func lexEscape(l *Lexer) stateFn {
 			return lexString
 		}
 		x = x*base + d
-		ch = l.next()
 		n--
+
+		// Don't seek after last rune.
+		if n > 0 {
+			ch = l.next()
+		}
 	}
 
 	if x > max || 0xD800 <= x && x < 0xE000 {

promql/parser/parse_test.go

@@ -3274,6 +3274,16 @@ var testSeries = []struct {
 		input:          `my_metric{a="b"} 1 2 3    `,
 		expectedMetric: labels.FromStrings(labels.MetricName, "my_metric", "a", "b"),
 		expectedValues: newSeq(1, 2, 3),
+	}, {
+		// Handle escaped unicode characters as whole label values.
+		input:          `my_metric{a="\u70ac"} 1 2 3`,
+		expectedMetric: labels.FromStrings(labels.MetricName, "my_metric", "a", `炬`),
+		expectedValues: newSeq(1, 2, 3),
+	}, {
+		// Handle escaped unicode characters as partial label values.
+		input:          `my_metric{a="\u70ac = torch"} 1 2 3`,
+		expectedMetric: labels.FromStrings(labels.MetricName, "my_metric", "a", `炬 = torch`),
+		expectedValues: newSeq(1, 2, 3),
 	}, {
 		input: `my_metric{a="b"} -3-3 -3`,
 		fail:  true,