Add warmup to EwmaRate

[jhalterman]

Describe the bug

The current math.EwmaRate implementation does not require a warmup period, allowing a rate to be available after a single sample is added, which can lead to unexpected results, as described in #5394. Let's move warmup support from that PR into EwmaRate itself, to avoid problems with other users of this type.

[samyakjain10]

Hi @jhalterman is the issue up for grab?

[jhalterman]

Yes it is.

[slimjim777]

@jhalterman the implementation in the PR compares the firstUpdate time with the sliding window, whereas the VividCortex approach is to keep a count of "warmup samples" and to compare with the WARMUP_SAMPLES const. Were you thinking that math.EwmaRate should use the current approach or switch to the count approach?

The count approach looks like a cleaner approach for math.EwmaRate, otherwise there would need to be a way to pass in the first update and sliding window times.