feat: add LGTM compose for the observability VM

WP-10(Terraform 설계서 6.10, 실험·앱 설계서 4.1). Mimir 3.2.1(monolithic, GCS
prefix blocks/ruler/alertmanager), Loki 3.7.8(tsdb v13, GCS), Tempo 3.0.3(OTLP
4317/4318, GCS), Grafana 13.2.2(데이터소스 Mimir·Loki·Tempo 프로비저닝). 오브젝트
스토리지는 VM SA의 ADC로 인증(키 파일 없음). Grafana 관리자 비밀번호는 Secret Manager
selfheal-grafana-admin에서 실행 시 취득. 알림 규칙 8건은 WP-14에서 추가.

vm/observability/loki.yaml

@@ -0,0 +1,37 @@
+# Loki single binary, TSDB index, GCS object store via the VM service account (ADC).
+auth_enabled: false
+server:
+  http_listen_port: 3100
+  grpc_listen_port: 9096
+  log_level: warn
+common:
+  path_prefix: /loki
+  replication_factor: 1
+  ring:
+    kvstore:
+      store: inmemory
+  storage:
+    gcs:
+      bucket_name: selfheal-cms-d-414903-loki
+schema_config:
+  configs:
+    - from: "2024-01-01"
+      store: tsdb
+      object_store: gcs
+      schema: v13
+      index:
+        prefix: index_
+        period: 24h
+storage_config:
+  tsdb_shipper:
+    active_index_directory: /loki/tsdb-index
+    cache_location: /loki/tsdb-cache
+compactor:
+  working_directory: /loki/compactor
+limits_config:
+  allow_structured_metadata: true
+ruler:
+  storage:
+    type: local
+    local:
+      directory: /loki/rules