More flexible load balancer selector

[aleanon]

Hi.

I am using pingora for an adaptive loadbalancer, but to avoid creating lots of different loadbalancers and routing between them I had to pull out the entire load-balancing crate and make changes to it.

So I am wondering if there is any interest in having a more flexible selector for the load balancer, specifically being able to conditionally build the selector.

The implementation I have in mind have no extra cost in the hot path. It only requires an extra field in the Load balancer struct that is not used in the hot path.

I just want to hear if there is any interest for this at all before creating a more detailed issue and PR.