docs: add Quickstart in observability; examples: add docker-compose.metrics.collector.yml and prometheus.with-collector.yml (collector-only scrape)

Former-commit-id: aea80200e002a31e60ba94da551411804b2d8db3

examples/prometheus.with-collector.yml

@@ -0,0 +1,17 @@
+global:
+  scrape_interval: 15s
+
+scrape_configs:
+  # WICHTIG: Newt NICHT direkt scrapen, nur den Collector!
+  - job_name: 'otel-collector'
+    static_configs:
+      - targets: ['otel-collector:8889']
+
+    # optional: Kardinalität begrenzen
+    relabel_configs:
+      - action: labeldrop
+        regex: 'tunnel_id'
+      # - action: keep
+      #   source_labels: [site_id]
+      #   regex: '(site-a|site-b)'
+