feature: DLQ 이벤트 모니터링 인터페이스 및 기본 구현체 추가

- `MonitoringService` 인터페이스 및 `DummyMonitoringAdapter` 구현체 추가
- DLQ 저장소에서 이벤트 저장 성공 시 메트릭 증가 로직 추가
- 이벤트 저장 실패 시 치명적 알림 로직 적용 (모니터링 서비스 활용)

Author: yeonjiyeon

apps/commerce-api/src/main/java/com/loopers/application/event/FailedEventStore.java

@@ -2,6 +2,7 @@
 
 import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.jackson.databind.ObjectMapper;
+import com.loopers.application.monitoring.port.MonitoringService;
 import com.loopers.domain.event.DomainEvent;
 import com.loopers.domain.event.FailedEvent;
 import com.loopers.infrastructure.event.FailedEventRepository;
@@ -16,6 +17,7 @@ public class FailedEventStore implements FailedEventScheduler {
 
   private final FailedEventRepository failedEventRepository;
   private final ObjectMapper objectMapper;
+  private final MonitoringService monitoringService;
 
   @Transactional
   @Override
@@ -38,7 +40,11 @@ public <T extends DomainEvent> void scheduleRetry(T event, String reason) {
 
     try {
       failedEventRepository.save(failedEvent);
+      monitoringService.incrementMetric("dlq.event_saved_total", "type:" + failedEvent.getEventType());
+
     } catch (Exception e) {
+      monitoringService.sendCriticalAlert(
+          "CRITICAL: DLQ 저장소 장애. 이벤트 유실 위험 발생! 사유: " + e.getMessage(), e);
     }
   }
 }

apps/commerce-api/src/main/java/com/loopers/application/monitoring/port/DummyMonitoringAdapter.java

@@ -0,0 +1,18 @@
+package com.loopers.application.monitoring.port;
+
+import lombok.extern.slf4j.Slf4j;
+import org.springframework.stereotype.Component;
+
+@Slf4j
+@Component
+public class DummyMonitoringAdapter implements MonitoringService {
+  @Override
+  public void incrementMetric(String name, String tag) {
+    log.info("[METRIC_DUMMY] Increment | Name: {} | Tag: {}", name, tag);
+  }
+
+  @Override
+  public void sendCriticalAlert(String message, Throwable t) {
+    log.error("[ALERT_DUMMY] CRITICAL ALERT: {}", message, t);
+  }
+}

apps/commerce-api/src/main/java/com/loopers/application/monitoring/port/MonitoringService.java

@@ -0,0 +1,8 @@
+package com.loopers.application.monitoring.port;
+
+public interface MonitoringService {
+
+  void incrementMetric(String name, String tag);
+
+  void sendCriticalAlert(String message, Throwable t);
+}