Expose transaction sample statistics through special key-space

The client transaction sample records a lot of interesting data about sampled transactions, but it does so in a binary format that I believe is undocumented and that isn't necessarily stable between versions. Additionally, any transformations of that data (say, to compute client latency percentiles) need to be implemented by all users of the data.

It's possible we could expose various interesting statistics based on this sample through the special key-space. I haven't thought about this extensively, but we could expose things like:

Latency percentiles
Error counts
Operation counts/rates