AvgOverTime can not work well

[bufferflies]

Bug Report

What did you do?

func TestPulse2(t *testing.T) {
	t.Parallel()
	re := require.New(t)
	dur := 5 * time.Second
	aot := NewAvgOverTime(dur)
	aot.Add(1000, dur)
	re.Equal(float64(1000), aot.GetInstantaneous())
	aot.Clear()
	aot.Add(1000, dur)
	re.Equal(float64(1000), aot.GetInstantaneous())
}

What did you expect to see?

GetInstantaneous is lastest value

What did you see instead?

GetInstantaneous is zero

What version of PD are you using (pd-server -V)?

master