Add demo of observability stack for monitoring Plano LLM gateway traffic

- Introduced Docker Compose setup with OpenTelemetry Collector, Tempo, Prometheus, and Grafana.
- Configured OTEL Collector to receive traces and derive Prometheus metrics.
- Added Grafana dashboards for visualizing LLM request metrics and latencies.
- Included configuration files for Prometheus and Tempo to support trace storage and metrics generation.
- Updated README with setup instructions and architecture overview.

demos/observability/grafana/provisioning/datasources/datasources.yaml

@@ -0,0 +1,20 @@
+apiVersion: 1
+
+datasources:
+  - name: Prometheus
+    type: prometheus
+    access: proxy
+    url: http://prometheus:9090
+    isDefault: true
+    editable: true
+
+  - name: Tempo
+    type: tempo
+    access: proxy
+    url: http://tempo:3200
+    editable: true
+    jsonData:
+      tracesToMetrics:
+        datasourceUid: Prometheus
+      serviceMap:
+        datasourceUid: Prometheus