PCR docs should provide more detail on how to scrape a node with virtual clusters

[dhartunian]

Can we add something to PCR docs on this page https://www.cockroachlabs.com/docs/v25.1/cluster-virtualization-metric-scopes that mentions that scraping _status/vars with the query param ?cluster=system is recommended in order to retrieve metrics for the node and the virtual cluster. This is necessary because if they set the server.controller.default_target_cluster cluster setting, this will only show that virtual cluster’s metrics in /_status/vars.

Jira issue: DOC-12932