In normal kubernetes metrics, you will find cAdvisor, Metrics Server, Kubernetes API Server, Node Exporter, and Kube-State-Metrics as source of metrics. In last 2 days, I noticed prometheus-adapter. Today KEDA popped up in my mind. I checked its website and found it can be source of metric as well. cAdvisor, Metrics Server, Kubernetes API Server, Node Exporter, and Kube-State-Metrics can be considered as in-cluster source of metric. However KEDA is about out-cluster source of metrics.