Least load balancer (#2999)

* v5: Health Check & LeastLoad Strategy (rebased from 2c5a71490368500a982018a74a6d519c7e121816)

Some changes will be necessary to integrate it into V2Ray

* Update proto

* parse duration conf with time.Parse()

* moving health ping to observatory as a standalone component

* moving health ping to observatory as a standalone component: auto generated file

* add initialization for health ping

* incorporate changes in router implementation

* support principle target output

* add v4 json support for BurstObservatory & fix balancer reference

* update API command

* remove cancelled API

* return zero length value when observer is not found

* remove duplicated targeted dispatch

* adjust test with updated structure

* bug fix for observer

* fix strategy selector

* fix strategy least load

* Fix ticker usage

ticker.Close does not close ticker.C

* feat: Replace default Health Ping URL to HTTPS (#1991)

* fix selectLeastLoad() returns wrong number of nodes (#2083)

* Test: fix leastload strategy unit test

* fix(router): panic caused by concurrent map read and write (#2678)

* Clean up code

---------

Co-authored-by: Jebbs <qjebbs@gmail.com>
Co-authored-by: Shelikhoo <xiaokangwang@outlook.com>
Co-authored-by: 世界 <i@sekai.icu>
Co-authored-by: Bernd Eichelberger <46166740+4-FLOSS-Free-Libre-Open-Source-Software@users.noreply.github.com>
Co-authored-by: 秋のかえで <autmaple@protonmail.com>
Co-authored-by: Rinka <kujourinka@gmail.com>

app/observatory/burst/ping.go

@@ -0,0 +1,69 @@
+package burst
+
+import (
+	"context"
+	"net/http"
+	"time"
+
+	"github.com/xtls/xray-core/common/net"
+	"github.com/xtls/xray-core/transport/internet/tagged"
+)
+
+type pingClient struct {
+	destination string
+	httpClient  *http.Client
+}
+
+func newPingClient(ctx context.Context, destination string, timeout time.Duration, handler string) *pingClient {
+	return &pingClient{
+		destination: destination,
+		httpClient:  newHTTPClient(ctx, handler, timeout),
+	}
+}
+
+func newDirectPingClient(destination string, timeout time.Duration) *pingClient {
+	return &pingClient{
+		destination: destination,
+		httpClient:  &http.Client{Timeout: timeout},
+	}
+}
+
+func newHTTPClient(ctxv context.Context, handler string, timeout time.Duration) *http.Client {
+	tr := &http.Transport{
+		DisableKeepAlives: true,
+		DialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
+			dest, err := net.ParseDestination(network + ":" + addr)
+			if err != nil {
+				return nil, err
+			}
+			return tagged.Dialer(ctxv, dest, handler)
+		},
+	}
+	return &http.Client{
+		Transport: tr,
+		Timeout:   timeout,
+		// don't follow redirect
+		CheckRedirect: func(req *http.Request, via []*http.Request) error {
+			return http.ErrUseLastResponse
+		},
+	}
+}
+
+// MeasureDelay returns the delay time of the request to dest
+func (s *pingClient) MeasureDelay() (time.Duration, error) {
+	if s.httpClient == nil {
+		panic("pingClient no initialized")
+	}
+	req, err := http.NewRequest(http.MethodHead, s.destination, nil)
+	if err != nil {
+		return rttFailed, err
+	}
+	start := time.Now()
+	resp, err := s.httpClient.Do(req)
+	if err != nil {
+		return rttFailed, err
+	}
+	// don't wait for body
+	resp.Body.Close()
+	return time.Since(start), nil
+}