Add observability module: Spring AI built-in meters and spans, cost per endpoint from token usage, Prometheus and Grafana dashboard

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01JXVi2GMQ7bR5EmbUFdDj7N

observability/src/main/resources/application.yml

@@ -0,0 +1,31 @@
+spring:
+  application:
+    name: spring-ai-observability
+  ai:
+    openai:
+      api-key: test-key-not-a-secret
+      base-url: ${FAKE_OPENAI_URL:http://localhost:8099}
+      chat:
+        options:
+          model: gpt-4o-mini
+management:
+  endpoints:
+    web:
+      exposure:
+        include: health,prometheus,metrics
+  tracing:
+    sampling:
+      probability: 1.0
+    export:
+      otlp:
+        enabled: false   # set true (and management.opentelemetry.tracing.export.otlp.endpoint) to ship spans to a Collector
+  otlp:
+    metrics:
+      export:
+        enabled: false   # Prometheus scrapes /actuator/prometheus instead
+ai:
+  # Illustrative prices in USD per million tokens. They are example numbers for the demo, not a current price list.
+  pricing:
+    models:
+      gpt-4o-mini: { input-per-million: 0.15, output-per-million: 0.60 }
+      gpt-4o: { input-per-million: 2.50, output-per-million: 10.00 }