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/downstream.yaml

@@ -0,0 +1,33 @@
+# A trivially small HTTP service the app's "downstream" health indicator calls. Scale it to zero to
+# simulate a dependency outage: kubectl -n demo scale deploy/downstream --replicas=0
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: downstream
+  namespace: demo
+spec:
+  replicas: 1
+  selector:
+    matchLabels: {app: downstream}
+  template:
+    metadata:
+      labels: {app: downstream}
+    spec:
+      # busybox httpd runs as PID 1 and ignores SIGTERM, so without this a scale-to-zero leaves the
+      # "outage" serving for the full 30 s grace period.
+      terminationGracePeriodSeconds: 2
+      containers:
+        - name: httpd
+          image: busybox:1.37
+          imagePullPolicy: Never
+          command: ["sh", "-c", "mkdir -p /www && echo ok > /www/index.html && exec httpd -f -p 8080 -h /www"]
+          ports: [{containerPort: 8080}]
+---
+apiVersion: v1
+kind: Service
+metadata:
+  name: downstream
+  namespace: demo
+spec:
+  selector: {app: downstream}
+  ports: [{port: 8080, targetPort: 8080}]