docs: update plan with hostname label documentation

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

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

@@ -10,6 +10,7 @@
 | 4. Set metadata on hosts | ✅ Complete | All relevant hosts configured |
 | 5. Update alert rules | ✅ Complete | Role-based filtering implemented |
 | 6. Labels for service targets | ✅ Complete | Host labels propagated to all services |
+| 7. Add hostname label | ✅ Complete | All targets have `hostname` label for easy filtering |
 
 **Hosts with metadata configured:**
 - `ns1`, `ns2`: `role = "dns"`, `labels.dns_role = "primary"/"secondary"`
@@ -19,6 +20,12 @@
 
 **Implementation complete.** Branch: `prometheus-scrape-target-labels`
 
+**Query examples:**
+- `{hostname="ns1"}` - all metrics from ns1 (any job/port)
+- `node_cpu_seconds_total{hostname="monitoring01"}` - specific metric by hostname
+- `up{role="dns"}` - all DNS servers
+- `up{tier="test"}` - all test-tier hosts
+
 ---
 
 ## Goal