allow estimate of polling interval for the first reset

In use-cases where a counter can be created dynamically in the middle of a polling-interval, the time delta should be the time since the last poll as the change in count was essentially 0 from that time to the first increment and therefore the creation of the counter. Otherwise the first interval will have a higher rate simply because it was created later in the interval. This change allows the resettable counter to be created with an estimated polling interval used only until the first reset call on the counter.