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/docker-compose.yaml

@@ -0,0 +1,50 @@
+services:
+  # OpenTelemetry Collector: receives traces from Plano, derives Prometheus
+  # metrics via the spanmetrics connector, and forwards traces to Tempo.
+  otel-collector:
+    image: otel/opentelemetry-collector-contrib:0.102.0
+    command: ["--config=/etc/otel-collector-config.yaml"]
+    volumes:
+      - ./otel-collector-config.yaml:/etc/otel-collector-config.yaml:ro
+    ports:
+      - "9317:4317"   # OTLP gRPC (Plano sends traces here)
+      - "8889:8889"   # Prometheus metrics endpoint (spanmetrics)
+    depends_on:
+      - tempo
+
+  tempo:
+    image: grafana/tempo:2.5.0
+    command: ["-config.file=/etc/tempo.yaml"]
+    volumes:
+      - ./tempo.yaml:/etc/tempo.yaml:ro
+    ports:
+      - "9200:3200"   # Tempo HTTP API
+
+  prometheus:
+    image: prom/prometheus:v2.53.0
+    command:
+      - "--config.file=/etc/prometheus/prometheus.yml"
+      - "--storage.tsdb.retention.time=7d"
+    volumes:
+      - ./prometheus.yaml:/etc/prometheus/prometheus.yml:ro
+    ports:
+      - "9190:9090"
+    extra_hosts:
+      - "host.docker.internal:host-gateway"
+    depends_on:
+      - otel-collector
+
+  grafana:
+    image: grafana/grafana:11.1.0
+    environment:
+      - GF_SECURITY_ADMIN_USER=admin
+      - GF_SECURITY_ADMIN_PASSWORD=admin
+      - GF_USERS_ALLOW_SIGN_UP=false
+    volumes:
+      - ./grafana/provisioning:/etc/grafana/provisioning:ro
+      - ./grafana/dashboards:/var/lib/grafana/dashboards:ro
+    ports:
+      - "9000:3000"
+    depends_on:
+      - prometheus
+      - tempo