feat: Add key cardinality testing feature

[kentquirk]

Which problem is this PR solving?

• We've seen issues where the key cardinality of things being fed to a sampler is longer than the sampler AdjustmentInterval. This gives loadgen the ability to generate fields that will only include all possible values once the generator's sample period has completed.

So mykey=/k50,60 will generate a field called mykey that will have values of cardinality 50, but not all possible values will be used until 60 seconds have passed.

Includes testing and benchmark code, as well as a readme update.

[MikeGoldsmith]

@kentquirk what's the status if this change? Is it something we still want to pursue?

[kentquirk]

Yes! I've been running it locally, but I'd like an approval from a team member, please. This was used to mimic some traffic like what we were seeing from customers that breaks the EMA sampler.