feat: Add GetSampleRateMulti(key, count)

[kentquirk]

Is your feature request related to a problem? Please describe.

The samplers in this repository are designed to be called for each span individually, and increment their counters with ++. But in Refinery, the samplers are called once per trace. Each trace accounts for a number of spans, but that's not included in the calculations. Thus, throughput sampling in particular is potentially very wrong.

Describe the solution you'd like

Add a new function that takes count as a parameter, and implement the existing GetSampleRate call in terms of that function, passing 1 as the second parameter.

Describe alternatives you've considered

Calling the samplers once per span and ignoring most of the results -- but that's an expensive alternative.

Additional context

[kentquirk]

Fixed by #53.