add Prometheus monitoring for Compose and Azure deploy

[JonathanPschl]

Closes #68

Summary

Adds a minimal Prometheus monitoring setup, wired up for both the local Docker
Compose stack and the Azure VM deployment. Each service exposes an HTTP metrics
endpoint, and a single Prometheus container scrapes them all.

Changes

Application

• Spring services (api-gateway, user-service, grocery-service): added
  spring-boot-starter-actuator + micrometer-registry-prometheus, exposed the
  health and prometheus actuator endpoints, tagged metrics with the service
  name. Served at /actuator/prometheus.
• gen-ai (FastAPI): added prometheus-fastapi-instrumentator, exposing /metrics.

Local (Docker Compose)

• New prom/prometheus:v2.54.1 service in compose.yaml on port 9090, with
  monitoring/prometheus.yml mounted and a prometheus-data volume.
• monitoring/prometheus.yml: scrape config for all four services.

Azure deployment

• Ansible (infra/ansible/roles/deploy/tasks/main.yml): copy
  monitoring/prometheus.yml to the VM.
• Terraform (infra/terraform/main.tf): open NSG port 9090 for
  dev-stage.

Docs

• README "Monitoring (Prometheus)" section.

How to test

Local:

1. docker compose up --build
2. Open http://localhost:9090/targets — all 5 jobs should be UP.
3. Generate a recipe in the app (http://localhost:8081).
4. Query http_server_requests_seconds_count (Spring) and http_requests_total
   (gen-ai) — counts across gateway → backends → gen-ai reconcile with the action.

Azure: after deploy, Prometheus UI is at http://<vm-public-ip>:9090.

[coderabbitai]

Caution

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feature/prometheus-monitoring

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[timn21]

Looks good