fix: Update dynamic samplers to use GetSampleRateMulti

[kentquirk]

Which problem is this PR solving?

• As it says in Refinery's throughput calculations are based on traces, not spans #636, throughput and dynamic sampler calculations have been based on the wrong numbers for some time. They've been counting traces, not spans, and yet everything about Honeycomb is based on span count. Most people think it works by counting spans and are then surprised when sample rates seem incorrect for their volume, and in fact, our documentation says that it works this way.

Short description of the changes

• This changes Refinery's dynamic and throughput sampler requests to use the span count, not the trace count, when calculating numbers for the dynamic samplers and throughput samplers.

This will need to be reflected in updated documentation.

Closes #636.

[TylerHelmuth]

This is an exciting change to help people use the windowedthroughput sampler to enforce their rate limit, but it is a significant breaking change right? We definitely want to highlight this change in the release.

Is there anything we can do in the config converter to detect users that are affected by this change and warn them to look at their rules configs closely?

[kentquirk]

I believe that I can make the config converter notice that and emit a warning. It's on the list.