Feat: add load balancer (#1437)

* add balancer
* add p2c balancer
* add http client selector filter

Co-authored-by: yuemoxi <99347745@qq.com>
Co-authored-by: chenzhihui <zhihui_chen@foxmail.com>

selector/node/direct/direct.go

@@ -0,0 +1,52 @@
+package direct
+
+import (
+	"context"
+	"sync/atomic"
+	"time"
+
+	"github.com/go-kratos/kratos/v2/selector"
+)
+
+const (
+	defaultWeight = 100
+)
+
+var (
+	_ selector.WeightedNode        = &node{}
+	_ selector.WeightedNodeBuilder = &Builder{}
+)
+
+// node is endpoint instance
+type node struct {
+	selector.Node
+
+	// last lastPick timestamp
+	lastPick int64
+}
+
+// Builder is direct node builder
+type Builder struct{}
+
+// Build create node
+func (*Builder) Build(n selector.Node) selector.WeightedNode {
+	return &node{Node: n, lastPick: 0}
+}
+
+func (n *node) Pick() selector.DoneFunc {
+	now := time.Now().UnixNano()
+	atomic.StoreInt64(&n.lastPick, now)
+	return func(ctx context.Context, di selector.DoneInfo) {}
+}
+
+// Weight is node effective weight
+func (n *node) Weight() float64 {
+	if n.InitialWeight() != nil {
+		return float64(*n.InitialWeight())
+	}
+	return defaultWeight
+}
+
+func (n *node) PickElapsed() time.Duration {
+	return time.Duration(time.Now().UnixNano() - atomic.LoadInt64(&n.lastPick))
+}