Reset condition in burst sampler

[crazy-pe]

Hello,

When mocking time for test purpose I found a bug in the reset condition of the burst sampler. Here is simple test to reproduce the bug:

func TestBurst(t *testing.T) {
	sampler := &BurstSampler{Burst: 1, Period: time.Second}

	t0 := time.Now()
	now := t0
	mockedTime := func() time.Time {
		return now
	}

	TimestampFunc = mockedTime
	defer func() { TimestampFunc = time.Now }()

	scenario := []struct {
		tm   time.Time
		want bool
	}{
		{t0, true},
		{t0.Add(time.Second - time.Nanosecond), false},
		{t0.Add(time.Second), true},
		{t0.Add(time.Second + time.Nanosecond), false},
	}

	for i, step := range scenario {
		now = step.tm
		got := sampler.Sample(NoLevel)
		if got != step.want {
			t.Errorf("step %d (t=%s): expect %t got %t", i, step.tm, step.want, got)
		}
	}
}

The fix is trivial. In the inc() function, replace the > comparison with >=.

func (s *BurstSampler) inc() uint32 {
	now := TimestampFunc().UnixNano()
	resetAt := atomic.LoadInt64(&s.resetAt)
	var c uint32
	if now > resetAt { // to be replaced by: if now >= resetAt
		c = 1
		atomic.StoreUint32(&s.counter, c)
		newResetAt := now + s.Period.Nanoseconds()
		reset := atomic.CompareAndSwapInt64(&s.resetAt, resetAt, newResetAt)
		if !reset {
			// Lost the race with another goroutine trying to reset.
			c = atomic.AddUint32(&s.counter, 1)
		}
	} else {
		c = atomic.AddUint32(&s.counter, 1)
	}
	return c
}

If it's ok for you I open a PR with the fix and the unit test.