[robustness tests] Consider adding Keys, Count to Kubernetes traffic generator

### What would you like to be added?

Kubernetes traffic generator should use `Count` of Kubernetes-Etcd interface and invoke `Range`s with `keys_only` to better simulate actual Kubernetes traffic.

It seems that `keys_only` `Range` calls will be unlikely in the future due to https://github.com/kubernetes/kubernetes/pull/133873.

Work in https://github.com/kubernetes/kubernetes/pull/132355 also replaced calls to `Count` so it is likely the work on this may no longer be needed/useful.

### Why is this needed?

Part of https://github.com/etcd-io/etcd/issues/15820 and https://github.com/etcd-io/etcd/issues/20182