feat(telemetry): integrate OpenTelemetry observability stack with health metrics

- Add OpenTelemetry SDK, OTLP exporter, Prometheus integration
- Implement connection tracking with active/total/disconnection metrics
- Add health endpoint with uptime and connection counts
- Integrate tracing spans for socket events and engine messages
- Add metrics collection for event handling duration
- Update health endpoint to include live runtime state
- Add graceful telemetry shutdown in main function
- Implement engine session active metrics tracking
- Add namespace-specific attributes to connection metrics
- Introduce message edit history retrieval endpoint
- Add scheduled message CRUD operations and dispatcher
- Update Socket.IO event registration with observability
- Refactor component update to remove dead code allowance
- Add comprehensive environment variables documentation
- Implement detailed development guidelines in AGENTS.md

.env.example

@@ -0,0 +1,71 @@
+# =============================================================================
+# imks — IM 实时消息服务 环境变量配置
+# 复制此文件为 .env 并修改相应值
+# =============================================================================
+
+# --- 部署模式 ---
+# Adapter 模式: "local" (单节点) | "redis" | "nats"
+IMKS_ADAPTER=local
+
+# 当前节点唯一标识（默认取主机名）
+# IMKS_SERVER_ID=imks-node-1
+
+# Redis 连接（IMKS_ADAPTER=redis 时必需）
+# IMKS_REDIS_URL=redis://localhost:6379
+
+# NATS 连接（IMKS_ADAPTER=nats 时必需）
+# IMKS_NATS_URL=nats://localhost:4222
+
+# --- WebTransport (QUIC) ---
+# 启用 WebTransport 服务（需要 TLS 证书）
+# IMKS_WT_ENABLED=false
+# IMKS_WT_PORT=3001
+# IMKS_WT_CERT_PATH=/path/to/cert.pem
+# IMKS_WT_KEY_PATH=/path/to/key.pem
+
+# --- 数据库 ---
+# PostgreSQL 连接字符串
+# DATABASE_URL=postgres://imks:password@localhost:5432/imks
+DATABASE_URL=postgres://localhost/imks
+
+# 连接池配置
+# DATABASE_MAX_CONNECTIONS=10
+# DATABASE_MIN_CONNECTIONS=2
+# DATABASE_CONNECT_TIMEOUT=30
+# DATABASE_IDLE_TIMEOUT=600
+
+# --- appks gRPC 连接 ---
+# appks 核心服务地址
+# APPKS_GRPC_ADDR=http://localhost:50051
+
+# 连接超时（秒）
+# APPKS_GRPC_TIMEOUT=10
+
+# mTLS 配置（生产环境必需）
+# APPKS_GRPC_TLS_CA_CERT=/path/to/ca.pem
+# APPKS_GRPC_TLS_CLIENT_CERT=/path/to/client.pem
+# APPKS_GRPC_TLS_CLIENT_KEY=/path/to/client-key.pem
+# APPKS_GRPC_TLS_DOMAIN=appks.internal
+
+# --- OpenTelemetry 可观测性 ---
+# 服务名
+# OTEL_SERVICE_NAME=imks
+# OTEL_SERVICE_VERSION=0.1.0
+
+# OTLP 收集器地址
+# OTEL_EXPORTER_OTLP_ENDPOINT=http://localhost:4317
+# 协议: grpc | http/protobuf
+# OTEL_EXPORTER_OTLP_PROTOCOL=grpc
+
+# 启用/禁用 telemetry
+# OTEL_TRACES_ENABLED=true
+# OTEL_METRICS_ENABLED=true
+# OTEL_LOGS_ENABLED=true
+
+# 日志级别: trace | debug | info | warn | error
+RUST_LOG=info
+# 日志格式: json | pretty
+# LOG_FORMAT=json
+
+# 部署环境标识
+# OTEL_RESOURCE_ATTRIBUTES_DEPLOYMENT=development