bug: fix slice init length (#40637) (#41515)

Signed-off-by: dufucun <dufuchun@sohu.com>
Co-authored-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Co-authored-by: Anderson Queiroz <anderson.queiroz@elastic.co>
(cherry picked from commit 092929b)

Co-authored-by: dufucun <dufuchun@sohu.com>

libbeat/processors/fingerprint/fingerprint_test.go

@@ -505,7 +505,7 @@ func nRandomEvents(num int) []beat.Event {
 	charsetLen := len(charset)
 	b := make([]byte, 200)
 
-	events := make([]beat.Event, num)
+	events := make([]beat.Event, 0, num)
 	for i := 0; i < num; i++ {
 		for j := range b {
 			b[j] = charset[prng.Intn(charsetLen)]

libbeat/processors/ratelimit/limit.go

@@ -52,7 +52,7 @@ func (l *rate) Unpack(str string) error {
 	}
 
 	if allowed := []unit{unitPerSecond, unitPerMinute, unitPerHour}; !contains(allowed, unitStr) {
-		allowedStrs := make([]string, len(allowed))
+		allowedStrs := make([]string, 0, len(allowed))
 		for _, a := range allowed {
 			allowedStrs = append(allowedStrs, "/"+string(a))
 		}