1. Proposed title of this feature request:

ETCD backup not including "external-loadbalancer-serving-certkey".
One customer encountered the issue of missing the external-load balancer cert and key.
~~~
E0919 07:12:56.374048 16 run.go:74] "command failed" err="failed to load SNI cert and key: missing content for serving cert \"sni-serving-cert::/etc/kubernetes/static-pod-certs/secrets/external-loadbalancer-serving-certkey/tls.crt::/etc/kubernetes/static-pod-certs/secrets/external-loadbalancer-serving-certkey/tls.key\""
I0919 07:12:56.377964 1 main.go:235] Termination finished with exit code 1
I0919 07:12:56.377982 1 main.go:188] Deleting termination lock file "/var/log/kube-apiserver/.terminating"
~~~
We tried to retrieve the certificate from etcd-backup but we didn't find it.
~~~
sh-5.1# cd kube-apiserver-pod-18/secrets/
etcd-client/ localhost-recovery-client-token/ localhost-recovery-serving-certkey/ webhook-authenticator/
~~~
Hence requesting to include the external-loadbalancer-serving-certkey in etcd. backup.