Enhanced sampling RFC

[lucperkins]

The current sampler transform supports constant sampling by specifying rate and allows for some limited "bucketing" by specifying a key_field (where each discrete value for that field is a new abstract "bucket" for events).

In a new RFC, we should discuss ways of enhancing the sampler transform to support:

1. Bucketing based on arbitrarily complex criteria, with Remap expressions as the vehicle
2. More robust key-based sampling by allowing for multiple keys
3. Dynamic sampling for buckets, i.e. adjusting the sample rate in accordance with traffic patterns