monitoring: always include tier label in scrape configs

Previously tier was only included if non-default (not "prod"), which
meant prod hosts had no tier label. This made the Grafana tier filter
only show "test" since "prod" never appeared in label_values().

Now tier is always included, so both "prod" and "test" appear in the
fleet dashboard tier selector.

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

lib/monitoring.nix

@@ -58,10 +58,9 @@ let
       };
 
   # Build effective labels for a host
-  # Always includes hostname; only includes tier/priority/role if non-default
+  # Always includes hostname and tier; only includes priority/role if non-default
   buildEffectiveLabels = host:
-    { hostname = host.hostname; }
-    // (lib.optionalAttrs (host.tier != "prod") { tier = host.tier; })
+    { hostname = host.hostname; tier = host.tier; }
     // (lib.optionalAttrs (host.priority != "high") { priority = host.priority; })
     // (lib.optionalAttrs (host.role != null) { role = host.role; })
     // host.labels;