devops(backend): expose Prometheus metrics endpoint + OTLP trace export from Spring Boot

- Add micrometer-registry-prometheus (BOM-managed) to expose /actuator/prometheus
- Add micrometer-tracing-bridge-otel (BOM-managed) for Micrometer → OTel tracing bridge
- Add opentelemetry-spring-boot-starter 2.27.0 (pinned — not in Spring Boot BOM)
- Move management to port 8081 so Prometheus scrapes directly inside archiv-net,
  bypassing both Caddy and Spring Security's session-authenticated filter chain
- Configure otel.service.name and OTLP endpoint (default localhost:4317 for CI safety)
- Set tracing sampling probability to 1.0 in base config; override via env var in compose
- Add OTEL_EXPORTER_OTLP_ENDPOINT + MANAGEMENT_TRACING_SAMPLING_PROBABILITY to docker-compose.yml
- Expose management port 8081 inside archiv-net for Prometheus scraping
- Disable trace export in application-test.yaml (probability: 0.0) for deterministic CI

OTLP export failures are non-fatal; app starts cleanly without Tempo running.
Closes #576

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/pom.xml

@@ -197,6 +197,25 @@
 			<artifactId>jsoup</artifactId>
 			<version>1.18.1</version>
 		</dependency>
+
+		<!-- Observability: Prometheus metrics scrape endpoint (version managed by Spring Boot BOM) -->
+		<dependency>
+			<groupId>io.micrometer</groupId>
+			<artifactId>micrometer-registry-prometheus</artifactId>
+		</dependency>
+
+		<!-- Observability: Micrometer → OpenTelemetry tracing bridge (version managed by Spring Boot BOM) -->
+		<dependency>
+			<groupId>io.micrometer</groupId>
+			<artifactId>micrometer-tracing-bridge-otel</artifactId>
+		</dependency>
+
+		<!-- Observability: OTel Spring Boot auto-instrumentation — NOT in Spring Boot BOM, pinned explicitly -->
+		<dependency>
+			<groupId>io.opentelemetry.instrumentation</groupId>
+			<artifactId>opentelemetry-spring-boot-starter</artifactId>
+			<version>2.27.0</version>
+		</dependency>
 	</dependencies>
 
 

backend/src/main/resources/application.yaml

@@ -45,9 +45,34 @@ server:
   forward-headers-strategy: native
 
 management:
+  server:
+    # Management port is separate from the app port so that:
+    # (a) Caddy never proxies /actuator/* (it only routes :8080 → the app port)
+    # (b) Prometheus scrapes backend:8081 directly inside archiv-net, not via Caddy
+    # (c) Spring Security's session-authenticated filter chain on :8080 never sees actuator requests
+    port: 8081
+  endpoints:
+    web:
+      exposure:
+        include: health,info,prometheus,metrics
+  endpoint:
+    prometheus:
+      enabled: true
   health:
     mail:
       enabled: false
+  tracing:
+    sampling:
+      probability: 1.0   # 100% in dev; override via MANAGEMENT_TRACING_SAMPLING_PROBABILITY in prod compose
+
+# OpenTelemetry trace export — failures are non-fatal (app starts cleanly without Tempo running)
+# The default http://localhost:4317 ensures CI compatibility when no observability stack is present.
+otel:
+  service:
+    name: familienarchiv-backend
+  exporter:
+    otlp:
+      endpoint: ${OTEL_EXPORTER_OTLP_ENDPOINT:http://localhost:4317}
 
 springdoc:
   api-docs:

backend/src/test/resources/application-test.yaml

@@ -13,3 +13,10 @@ spring:
     password: test
   mail:
     host: localhost
+
+# Disable trace export in tests — prevents OTLP connection attempts when no Tempo is running.
+# Sampling probability 0.0 means no spans are created, so no export is attempted.
+management:
+  tracing:
+    sampling:
+      probability: 0.0

docker-compose.yml

@@ -147,8 +147,18 @@ services:
       SPRING_MAIL_PROPERTIES_MAIL_SMTP_STARTTLS_ENABLE: ${MAIL_STARTTLS_ENABLE:-false}
       APP_OCR_BASE_URL: http://ocr-service:8000
       APP_OCR_TRAINING_TOKEN: "${OCR_TRAINING_TOKEN:-}"
+      # Observability: send traces to Tempo inside archiv-net (OTLP gRPC port 4317)
+      # Tempo is defined in docker-compose.observability.yml (future issue).
+      # OTLP failures are non-fatal — backend starts cleanly without the observability stack.
+      OTEL_EXPORTER_OTLP_ENDPOINT: http://tempo:4317
+      # 10% sampling in this compose (dev + staging) — override locally to 1.0 if needed
+      MANAGEMENT_TRACING_SAMPLING_PROBABILITY: "0.1"
     ports:
       - "${PORT_BACKEND}:8080"
+      # Management port — Prometheus scrapes /actuator/prometheus from inside archiv-net.
+      # Not exposed to the host; Docker service-name DNS (backend:8081) is sufficient.
+    expose:
+      - "8081"
     networks:
       - archiv-net
     healthcheck: