Support metrics for spoke clusters

We need a way to show metrics for workloads running on spoke clusters. This depends on ACM-876, which lets the console discover the monitoring endpoints.

• Console operator must discover the external URLs for monitoring
• Console operator must pass the URLs and CA files as part of the cluster config to the console backend
• Console backend must set up proxies for each endpoint (as it does for the API server endpoints)
• Console frontend must include the cluster in metrics requests

Open Issues:

We will depend on ACM to create a route on each spoke cluster for the prometheus tenancy service, which is required for metrics for normal users.

Openshift console backend should proxy managed cluster monitoring requests through the MCE cluster proxy addon to prometheus services on the managed cluster. This depends on https://issues.redhat.com/browse/ACM-1188