grafana: fix fleet table __name__ columns

Exclude the __name__ columns that were leaking through the
table transformations.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

services/grafana/dashboards/nixos-fleet.json

@@ -298,6 +298,12 @@
               "Time 4": true,
               "Time 5": true,
               "Value #info": true,
+              "__name__": true,
+              "__name__ 1": true,
+              "__name__ 2": true,
+              "__name__ 3": true,
+              "__name__ 4": true,
+              "__name__ 5": true,
               "dns_role": true,
               "dns_role 1": true,
               "dns_role 2": true,