feat: Phase 2-5 监控中心全栈实现 — 指标浏览器/SNMP/站点拨测/中间件

Phase 2 指标监控:
- 修复 /metrics 路由(原指向 AlertManager → 现指向 MetricsExplorer)
- 新建 MetricsHandler (4 API): targets/list/query/names
- 新建 MetricsExplorer 前端(170行): 目标选择器+指标分类树+多指标叠加SVG图表

Phase 3 SNMP设备:
- 新建 SNMPHandler (9 API): devices CRUD/templates/stats
- SNMPMonitor 接入真实API(428行): 设备表格+添加对话框+删除

Phase 4 站点拨测:
- 新建 SiteHandler (8 API): tasks CRUD/results/stats
- SiteMonitor 接入真实API(542行): 任务表+添加对话框+拨测类型网格
- 新增 siteApi 到 api.ts

Phase 5 中间件采集:
- Agent 新增 collector/middleware.go: Redis INFO + MySQL handshake 采集
- Agent CLI 新增 -redis/-redis-auth/-mysql/-mysql-user/-mysql-pass
- MiddlewareMonitor 接入真实API(301行): 实例卡片+指标面板

基础设施:
- AutoMigrate 新增 23 张监控表
- 前后端容器构建部署验证通过

backend/internal/api/handlers/metrics.go

@@ -0,0 +1,132 @@
+package handlers
+
+import (
+	"ezops/internal/models"
+	"net/http"
+	"strconv"
+	"time"
+
+	"github.com/gin-gonic/gin"
+	"gorm.io/gorm"
+)
+
+// MetricsHandler 指标浏览器 handler // Logan.Li
+type MetricsHandler struct {
+	db *gorm.DB
+}
+
+// NewMetricsHandler 创建指标 handler
+func NewMetricsHandler(db *gorm.DB) *MetricsHandler {
+	return &MetricsHandler{db: db}
+}
+
+// ListTargets 列出监控目标（按类型过滤）
+// GET /api/v1/metrics/targets?type=host&status=online
+func (h *MetricsHandler) ListTargets(c *gin.Context) {
+	var targets []models.MonitorTarget
+	q := h.db.Model(&models.MonitorTarget{})
+
+	if t := c.Query("type"); t != "" {
+		q = q.Where("type = ?", t)
+	}
+	if s := c.Query("status"); s != "" {
+		q = q.Where("status = ?", s)
+	}
+	if kw := c.Query("keyword"); kw != "" {
+		q = q.Where("name LIKE ? OR ip LIKE ?", "%"+kw+"%", "%"+kw+"%")
+	}
+
+	// 分页
+	page, _ := strconv.Atoi(c.DefaultQuery("page", "1"))
+	pageSize, _ := strconv.Atoi(c.DefaultQuery("page_size", "50"))
+	var total int64
+	q.Count(&total)
+	q.Offset((page - 1) * pageSize).Limit(pageSize).Order("name ASC").Find(&targets)
+
+	if targets == nil {
+		targets = []models.MonitorTarget{}
+	}
+	// 附带最新指标值
+	type TargetWithMetrics struct {
+		models.MonitorTarget
+		LatestMetrics map[string]float64 `json:"latest_metrics"`
+	}
+	result := make([]TargetWithMetrics, len(targets))
+	for i, t := range targets {
+		result[i] = TargetWithMetrics{MonitorTarget: t, LatestMetrics: make(map[string]float64)}
+		// 查询该目标最近 10 分钟内的最新指标
+		var histories []models.MetricHistory
+		h.db.Where("target_id = ? AND timestamp > ?", t.ID, now()-600).
+			Order("timestamp DESC").Limit(10).Find(&histories)
+		for _, mh := range histories {
+			if _, ok := result[i].LatestMetrics[mh.MetricName]; !ok {
+				result[i].LatestMetrics[mh.MetricName] = mh.Value
+			}
+		}
+	}
+
+	c.JSON(http.StatusOK, models.Response{Code: 200, Data: gin.H{
+		"list":  result,
+		"total": total,
+	}})
+}
+
+// ListMetrics 列出系统中所有指标元数据
+// GET /api/v1/metrics/list
+func (h *MetricsHandler) ListMetrics(c *gin.Context) {
+	var metrics []models.MonitorMetric
+	h.db.Order("category ASC, name ASC").Find(&metrics)
+	if metrics == nil {
+		metrics = []models.MonitorMetric{}
+	}
+	c.JSON(http.StatusOK, models.Response{Code: 200, Data: metrics})
+}
+
+// QueryValues 查询指标时序数据
+// GET /api/v1/metrics/query?target_id=N&metric_name=cpu_usage&from=0&to=0&limit=100
+func (h *MetricsHandler) QueryValues(c *gin.Context) {
+	var histories []models.MetricHistory
+	q := h.db.Model(&models.MetricHistory{})
+
+	if tid := c.Query("target_id"); tid != "" {
+		id, _ := strconv.ParseUint(tid, 10, 64)
+		q = q.Where("target_id = ?", id)
+	}
+	if mn := c.Query("metric_name"); mn != "" {
+		q = q.Where("metric_name = ?", mn)
+	}
+	if from := c.Query("from"); from != "" {
+		ts, _ := strconv.ParseInt(from, 10, 64)
+		q = q.Where("timestamp >= ?", ts)
+	}
+	if to := c.Query("to"); to != "" {
+		ts, _ := strconv.ParseInt(to, 10, 64)
+		q = q.Where("timestamp <= ?", ts)
+	}
+
+	limit, _ := strconv.Atoi(c.DefaultQuery("limit", "200"))
+	q.Order("timestamp ASC").Limit(limit).Find(&histories)
+
+	if histories == nil {
+		histories = []models.MetricHistory{}
+	}
+	c.JSON(http.StatusOK, models.Response{Code: 200, Data: histories})
+}
+
+// ListMetricNames 列出所有可用的指标名称（去重）
+// GET /api/v1/metrics/names
+func (h *MetricsHandler) ListMetricNames(c *gin.Context) {
+	var names []string
+	h.db.Model(&models.MetricHistory{}).
+		Distinct("metric_name").
+		Order("metric_name ASC").
+		Pluck("metric_name", &names)
+	if names == nil {
+		names = []string{}
+	}
+	c.JSON(http.StatusOK, models.Response{Code: 200, Data: names})
+}
+
+func now() int64 {
+	return time.Now().Unix()
+}