Feature request: kubeadm certs check-expiration output in JSON or YAML format

[aborrero]

Thanks for your work with kubeadm, really appreciated.

We would like to get a JSON or YAML output of the certificate expiration information.

user@k8s-control:~$ sudo kubeadm certs check-expiration
[check-expiration] Reading configuration from the cluster...
[check-expiration] FYI: You can look at this config file with 'kubectl -n kube-system get cm kubeadm-config -o yaml'

CERTIFICATE                EXPIRES                  RESIDUAL TIME   CERTIFICATE AUTHORITY   EXTERNALLY MANAGED
admin.conf                 Apr 09, 2024 09:17 UTC   348d            ca                      no      
apiserver                  Apr 09, 2024 09:16 UTC   348d            ca                      no      
apiserver-kubelet-client   Apr 09, 2024 09:16 UTC   348d            ca                      no      
controller-manager.conf    Apr 09, 2024 09:16 UTC   348d            ca                      no      
front-proxy-client         Apr 09, 2024 09:16 UTC   348d            front-proxy-ca          no      
scheduler.conf             Apr 09, 2024 09:17 UTC   348d            ca                      no      

CERTIFICATE AUTHORITY   EXPIRES                  RESIDUAL TIME   EXTERNALLY MANAGED
ca                      Nov 03, 2029 14:13 UTC   6y              no      
front-proxy-ca          Nov 03, 2029 14:13 UTC   6y              no  

So we can easily parse the information from this output for another software (like, for example, alert if certs are about to expire).

[neolit123]

this seems like a good suggestion. thank you

added it to the main issue for machine readable output:
#494

and closing this one

cc @pacoxu @SataQiu

[neolit123]

@aborrero also if you or someone is willing to contribute the change, help is welcome.