fix(obs): disable OTLP metric export (Prometheus scrapes pull-model)

Tempo only handles traces; sending metrics to /v1/metrics returns 404.
Prometheus already scrapes Spring Boot metrics via the pull-model at
/actuator/prometheus, so OTLP metric push is redundant and noisy.

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

backend/src/main/resources/application.yaml

@@ -86,7 +86,9 @@ otel:
     otlp:
       endpoint: ${OTEL_EXPORTER_OTLP_ENDPOINT:http://localhost:4318}
   logs:
-    exporter: none   # Promtail captures Docker logs; disable OTLP log export to Tempo (which doesn't accept logs)
+    exporter: none   # Promtail captures Docker logs; disable OTLP log export (Tempo only accepts traces)
+  metrics:
+    exporter: none   # Prometheus scrapes /actuator/prometheus; disable OTLP metric export to Tempo
 
 springdoc:
   api-docs:

docker-compose.prod.yml

@@ -215,6 +215,7 @@ services:
       SPRING_MAIL_PROPERTIES_MAIL_SMTP_STARTTLS_ENABLE: ${MAIL_STARTTLS_ENABLE:-true}
       OTEL_EXPORTER_OTLP_ENDPOINT: http://tempo:4318
       OTEL_LOGS_EXPORTER: none
+      OTEL_METRICS_EXPORTER: none
       MANAGEMENT_METRICS_TAGS_APPLICATION: Familienarchiv
       MANAGEMENT_TRACING_SAMPLING_PROBABILITY: ${MANAGEMENT_TRACING_SAMPLING_PROBABILITY:-0.1}
     networks: