Add kubernetes-deployment: probes, shutdown, JVM ergonomics, HPA

Companion code for "Deploying Spring Boot 4 on Kubernetes: Probes, Graceful
Shutdown, Limits and JVM Ergonomics". A dependency outage under three
probe-group setups, a rolling restart under load four ways (three runs
each), the JVM's ergonomic choices for nine pod shapes, one GC-heavy load
under five CPU limits with throttling counters, and an HPA driven by a
Micrometer gauge through prometheus-adapter. Measured on k3s v1.36.4.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01C3TETMrqVUWeFkNtz3Jbo3

kubernetes-deployment/k8s/hpa/hpa.yaml

@@ -0,0 +1,20 @@
+# Scale orders on in-flight requests per pod, not CPU. Target: 5 in flight on average.
+apiVersion: autoscaling/v2
+kind: HorizontalPodAutoscaler
+metadata: {name: orders, namespace: demo}
+spec:
+  scaleTargetRef: {apiVersion: apps/v1, kind: Deployment, name: orders}
+  minReplicas: 1
+  maxReplicas: 4
+  metrics:
+    - type: Pods
+      pods:
+        metric: {name: app_inflight_requests}
+        target: {type: AverageValue, averageValue: "5"}
+  behavior:
+    scaleUp:
+      stabilizationWindowSeconds: 0
+    scaleDown:
+      # The default is 300 s. Shortened so the demo shows a scale-down inside a few minutes;
+      # keep the default (or longer) in production.
+      stabilizationWindowSeconds: 30