Where are the possible metrics for kubernetes autoscaling defined
CPU or Memory usage in ResourceMetric is provided by kubelet and collected by metric-server
But for packets-per-second and requests-per-second, there are no official provider, so this field can actually be any value, depend on the non-official custom metrics API you deployed.
Some popular custom metrics API are listed at https://github.com/kubernetes/metrics/blob/master/IMPLEMENTATIONS.md
The GitHub project below provides a lot of information about using custom metrics provided by Prometheus for autoscaling of Pods in Kubernetes.