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-servicemonitor.yaml

@@ -0,0 +1,15 @@
+apiVersion: monitoring.coreos.com/v1
+kind: ServiceMonitor
+metadata:
+  name: pingora
+  namespace: ingress
+  labels:
+    app: pingora
+spec:
+  selector:
+    matchLabels:
+      app: pingora
+  endpoints:
+    - port: metrics
+      interval: 15s
+      path: /metrics