Max pool group size

[spacebat]

A feature that would be useful to us is to put a limit on the combined size of all the pools in a group. Our use case is kubernetes pods - we only want to run up to N of them overall, but we have a pool of pods for each image that we run. It would be nice if each pool gets its init_size guaranteed, but can only grow if the total number of workers across all pools in the group has not exceeded N. That way a large workload that only uses one image could run something close to N workers, and when that workload is finished there would be more slots avialable for pods that use other images.

Just sketching the idea here as I've yet to try out Pooler - we're currently using Poolex which lacks the grace period to retire extra workers - it's that feature that has me looking into Pooler.