fix rate limiter bug

[zwkno1]

i write this code to test rate limiter:

func TestRateLimit(t *testing.T) {
	limiter := rate.NewLimiter(5, 5)

	wait := sync.WaitGroup{}

	var count uint64
	startTime := time.Now().Unix()

	go func() {
		for {
			elapsed := time.Now().Unix() - startTime
			var qps uint64
			if elapsed > 0 {
				qps = atomic.LoadUint64(&count) / uint64(elapsed)
			}
			t.Log("qps: ", qps)

			time.Sleep(time.Second)
		}
	}()

	for i := 0; i < 100000; i++ {
		wait.Add(1)
		go func() {
			for {
				if limiter.Allow() {
					atomic.AddUint64(&count, 1)
				}
				time.Sleep(time.Microsecond * 10)
			}
			wait.Done()
		}()
	}
	wait.Wait()
}

the test outputs:
TestRateLimit: ratelimiter_test.go:32: qps: 0
TestRateLimit: ratelimiter_test.go:32: qps: 91541
TestRateLimit: ratelimiter_test.go:32: qps: 199775
TestRateLimit: ratelimiter_test.go:32: qps: 228304
TestRateLimit: ratelimiter_test.go:32: qps: 246161
TestRateLimit: ratelimiter_test.go:32: qps: 260489

there is a bug caused by timestamp before last

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