The /metrics endpoint at kubelet is exposed by the kubelet itself. Kubelet collects the metrics information from the node it is running. Relevant information can be found in the k8s documentations(https://kubernetes.io/docs/concepts/cluster-administration/system-metrics/). Metrics in Kubernetes.

When calling the /metrics called at api-server, it will collect information from all relevant components, including the /metrics from kubelet, which can be shown in the Resource metrics pipeline(https://kubernetes.io/docs/tasks/debug/debug-cluster/resource-metrics-pipeline/#:~:text=The%20metrics%2Dserver%20calls%20the,metrics%2Fresource%20in%20version%20v0.). Resource metrics pipeline.

Go back to your question, kubelet on each node expose /metrics endpoint, the statistics from which are maintained by kubelet itself.