Add observatory / latestPing balancing strategy

Co-authored-by: Shelikhoo <xiaokangwang@outlook.com>

app/router/strategy_leastping.go

@@ -0,0 +1,57 @@
+package router
+
+import (
+	"context"
+	"github.com/xtls/xray-core/core"
+
+	"github.com/xtls/xray-core/app/observatory"
+	"github.com/xtls/xray-core/common"
+	"github.com/xtls/xray-core/features/extension"
+)
+
+type LeastPingStrategy struct {
+	ctx         context.Context
+	observatory extension.Observatory
+}
+
+func (l *LeastPingStrategy) InjectContext(ctx context.Context) {
+	common.Must(core.RequireFeatures(ctx, func(observatory extension.Observatory) error {
+		l.observatory = observatory
+		return nil
+	}))
+	l.ctx = ctx
+}
+
+func (l *LeastPingStrategy) PickOutbound(strings []string) string {
+	observeReport, err := l.observatory.GetObservation(l.ctx)
+	if err != nil {
+		newError("cannot get observe report").Base(err).WriteToLog()
+		return ""
+	}
+	outboundsList := outboundList(strings)
+	if result, ok := observeReport.(*observatory.ObservationResult); ok {
+		status := result.Status
+		leastPing := int64(99999999)
+		selectedOutboundName := ""
+		for _, v := range status {
+			if outboundsList.contains(v.OutboundTag) && v.Alive && v.Delay < leastPing {
+				selectedOutboundName = v.OutboundTag
+			}
+		}
+		return selectedOutboundName
+	}
+
+	//No way to understand observeReport
+	return ""
+}
+
+type outboundList []string
+
+func (o outboundList) contains(name string) bool {
+	for _, v := range o {
+		if v == name {
+			return true
+		}
+	}
+	return false
+}