docs: add dns_role label to scrape target labels plan

Add proposed dns_role label to distinguish primary/secondary DNS
resolvers. This addresses the unbound_low_cache_hit_ratio alert
firing on ns2, which has a cold cache due to low traffic.

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

docs/plans/prometheus-scrape-target-labels.md

@@ -32,6 +32,24 @@ Values: free-form string, e.g. `"dns"`, `"build-host"`, `"database"`, `"monitori
 
 Recommendation: start with a single primary role string. If multi-role matching becomes a real need, switch to separate boolean labels.
 
+### `dns_role`
+
+For DNS servers specifically, distinguish between primary and secondary resolvers. The secondary resolver (ns2) receives very little traffic and has a cold cache, making generic cache hit ratio alerts inappropriate.
+
+Values: `"primary"`, `"secondary"`
+
+Example use case: The `unbound_low_cache_hit_ratio` alert fires on ns2 because its cache hit ratio (~62%) is lower than ns1 (~90%). This is expected behavior since ns2 gets ~100x less traffic. With a `dns_role` label, the alert can either exclude secondaries or use different thresholds:
+
+```promql
+# Only alert on primary DNS
+unbound_cache_hit_ratio < 0.7 and on(instance) unbound_up{dns_role="primary"}
+
+# Or use different thresholds
+(unbound_cache_hit_ratio < 0.7 and on(instance) unbound_up{dns_role="primary"})
+or
+(unbound_cache_hit_ratio < 0.5 and on(instance) unbound_up{dns_role="secondary"})
+```
+
 ## Implementation
 
 ### 1. Add `labels` option to `homelab.monitoring`