fix(promtail): move monitoring drop from relabel to pipeline_stages

relabel drop removed the entire stream before labels were set, causing
the "at least one label pair required" error. pipeline_stages drop runs
after labels are established, which is correct for filtering by tier.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docker/promtail/config.yaml

@@ -82,12 +82,11 @@ scrape_configs:
         target_label: "tier"
         replacement: "other"
 
-      # Drop monitoring container logs to save space (they're noisy)
-      - source_labels: ["tier"]
-        regex: "monitoring"
-        action: drop
-
     pipeline_stages:
+      # Drop monitoring container logs to save space (they're noisy)
+      - match:
+          selector: '{tier="monitoring"}'
+          action: drop
       # Try to parse JSON logs (Go services, Hono services)
       - json:
           expressions: