ExpAvgMeasurement doesn't properly handle add after update during warmup

[jhalterman]

I noticed that ExpAvgMeasurement doesn't properly handle an add after an update when still in warmup mode. Here's a failing test:

  @Test
  public void testUpdateThenAdd() {
    ExpAvgMeasurement avg = new ExpAvgMeasurement(100, 10);
    avg.update(i -> 50);
    avg.add(100);

    Assert.assertEquals(67, avg.get().doubleValue(), 0.1);
  }

It might make sense to consider the warmup phase done once any update happens.