feat(metrics): NEWT_METRICS_INCLUDE_TUNNEL_ID toggle; conditionally drop tunnel_id across bytes/sessions/proxy/reconnect; docs and smoke test updated; examples/prometheus.yml with relabels; docker-compose defaults avoid double-scrape

Former-commit-id: b20f7a02b2fe861d27111f8c7716a225866ee025

examples/prometheus.yml

@@ -2,9 +2,20 @@ global:
   scrape_interval: 15s
 
 scrape_configs:
-  - job_name: newt
+  - job_name: 'newt'
+    scrape_interval: 15s
     static_configs:
-      - targets: ["newt:2112"]
-  - job_name: otel-collector
-    static_configs:
-      - targets: ["collector:8889"]
+      - targets: ['newt:2112']  # /metrics
+    relabel_configs:
+      # optional: tunnel_id droppen
+      - action: labeldrop
+        regex: 'tunnel_id'
+      # optional: nur bestimmte sites zulassen
+      - action: keep
+        source_labels: [site_id]
+        regex: '(site-a|site-b)'
+
+  # WARNING: Do not enable this together with the 'newt' job above or you will double-count.
+  # - job_name: 'otel-collector'
+  #   static_configs:
+  #     - targets: ['collector:8889']