Tail Sampler: Improve Rate Limit Algorithm

[chris-smith-zocdoc]

Describe the solution you'd like
The current rate_limiting algorithm enforces a strict spans per second limit which has a couple of flaws:

• Traces with more spans than the span/sec limit cannot be sampled, they are always dropped
• Variations in the arrival rate of spans cannot be used to "burst" and temporarily consume previously unused capacity. This is desirable when the exact spans/sec rate is not required and the capacity can be spread over minutes or hours.

Describe alternatives you've considered
A token bucket or leaky bucket implementation would be useful

[github-actions]

This issue has been inactive for 60 days. It will be closed in 120 days if there is no activity.