Checkout the official Kubernetes documentation links:
You need to create a secret that contains the ETCD healthcheck client cert and key.
kubectl \
create \
-n monitoring \
secret generic \
etcd-client-cert \
--from-file=/etc/kubernetes/pki/etcd/ca.crt \
--from-file=/etc/kubernetes/pki/etcd/healthcheck-client.crt \
--from-file=/etc/kubernetes/pki/etcd/healthcheck-client.key
For more information checkout this comment: https://github.com/prometheus-community/helm-charts/issues/204#issuecomment-765155883