don't flush null values

  def flush_every(interval: Long, proc: (Long, Double) => Unit) = {

    val now = FnordMetric.now

    var triggered = (next_flush == 0)

    if (next_flush == 0)

    if (triggered)

      next_flush = now

    while (next_flush <= now) {

      if (!triggered)

        proc(next_flush, flush)

      next_flush += interval

      triggered = true

    }

  }