[FEATURE] Support Get certificates API for cluster-metric monitors

[machenity]

Is your feature request related to a problem?

I tried to monitor the expiration datetimes of the registered TLS certificates and raise alerts before their not_after is 30 days left. I considered to use cluster-metric monitor but it seems get certificates API is not supported now. Only these APIs are supported.

What solution would you like? It would be great to be able to use Certificates API for cluster-metric monitors.