feat(observability): enable OTLP tracing, fix Prometheus scraping, add proxy ServiceMonitor

- Set otlp_endpoint to Tempo HTTP receiver (port 4318) for request tracing
- Add hostNetwork to prometheusSpec so it can reach kubelet/node-exporter on node public IP
- Add ServiceMonitor for proxy metrics scrape on port 9090
- Add CORS origin and Grafana datasource config for monitoring stack

base/ingress/pingora-config.yaml

@@ -21,8 +21,7 @@ data:
     key_path  = "/etc/tls/tls.key"
 
     [telemetry]
-    # Empty = OTEL disabled. Set to http://otel-collector.data.svc:4318 when ready.
-    otlp_endpoint = ""
+    otlp_endpoint = "http://tempo.monitoring.svc.cluster.local:4318"
     metrics_port  = 9090
 
     # Kubernetes resource names for cert/config watchers.