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/mimir.yaml

@@ -0,0 +1,47 @@
+# Mimir monolithic (target=all), single node, GCS backend via the VM service account (ADC; no key file).
+# One bucket, separate prefixes: blocks must not share bucket+prefix with ruler/alertmanager stores.
+target: all
+multitenancy_enabled: false
+server:
+  http_listen_port: 9009
+  grpc_listen_port: 9095
+  log_level: warn
+common:
+  storage:
+    backend: gcs
+    gcs:
+      bucket_name: selfheal-cms-d-414903-mimir
+blocks_storage:
+  storage_prefix: blocks
+  tsdb:
+    dir: /data/tsdb
+  bucket_store:
+    sync_dir: /data/tsdb-sync
+ruler_storage:
+  storage_prefix: ruler
+alertmanager_storage:
+  storage_prefix: alertmanager
+ruler:
+  rule_path: /data/ruler
+alertmanager:
+  data_dir: /data/alertmanager
+  external_url: http://10.20.0.10:9009/alertmanager
+compactor:
+  data_dir: /data/compactor
+  sharding_ring:
+    kvstore:
+      store: memberlist
+distributor:
+  ring:
+    kvstore:
+      store: memberlist
+ingester:
+  ring:
+    kvstore:
+      store: memberlist
+    replication_factor: 1
+store_gateway:
+  sharding_ring:
+    replication_factor: 1
+memberlist:
+  join_members: []