Enable metrics does not trigger hook

Hi all,

I've tried to enable metrics server on my CDK but pod keeps crashing with following log:

I0710 13:53:55.429053 1 serving.go:273] Generated self-signed cert (apiserver.local.config/certificates/apiserver.crt, apiserver.local.config/certificates/apiserver.key)
W0710 13:54:10.329040 1 authentication.go:166] cluster doesn't provide client-ca-file in configmap/extension-apiserver-authentication in kube-system, so client certificate authentication to extension api-server won't work.
Error: cluster doesn't provide requestheader-client-ca-file

I think it is related to requestheader-* kube-apiserver args
https://github.com/charmed-kubernetes/charm-kubernetes-master/commit/29a44d19ebb418b8e676d16946c40fbc58dae3ce as they are not created.

hookenv.config('enable-metrics') should trigger api_opts but I don't see those args on kube-apiserver and also, configmap extension-apiserver-authentication in kube-system has no data.

On Slack, cynerva found this:

I'm looking at the charm code and I don't see a handler for the `config.changed.enable-metrics` event. That's probably why it's not restarting the apiserver with the correct opts.

After I ran these two commands, metrics server started to work and HPA also works.

juju run --application kubernetes-master -- charms.reactive clear_flag kubernetes-master.components.started
juju run --application kubernetes-master -- hooks/update-status

Kubernetes 1.14
Kubernetes-master charm 695

Thanks