Use alternative labels in variable drop downs

We’re using a node exporter on various instances and have a dashboard for a node. The instance is being choosed in Grafana by a variable, type query (label_values(node_uname_info, instance)). We want to assign human readable names instead of the rather cryptical instance dns names. How can we do that?

Out best try currently is to assign each target a custom label (name: “foobar”) in the static_configs of prometheus. But this leads to the following problem: When a label changes, the old label stays in the dropdown, since there are (old) values in prometheus with the old label.
Can we only show labels from the last scrape? Or only label values occurring in the last n minutes?