deduplicate common logic across proxy, visitor, and metrics modules (#5213)

- Replace duplicate parseBasicAuth with existing httppkg.ParseBasicAuth
- Extract buildDomains helper in BaseProxy for HTTP/HTTPS/TCPMux proxies
- Extract toProxyStats helper to deduplicate ProxyStats construction
- Extract startVisitorListener helper in BaseProxy for STCP/SUDP proxies
- Extract acceptLoop helper in BaseVisitor for STCP/XTCP visitors

server/proxy/stcp.go

@@ -41,21 +41,7 @@ func NewSTCPProxy(baseProxy *BaseProxy) Proxy {
 }
 
 func (pxy *STCPProxy) Run() (remoteAddr string, err error) {
-	xl := pxy.xl
-	allowUsers := pxy.cfg.AllowUsers
-	// if allowUsers is empty, only allow same user from proxy
-	if len(allowUsers) == 0 {
-		allowUsers = []string{pxy.GetUserInfo().User}
-	}
-	listener, errRet := pxy.rc.VisitorManager.Listen(pxy.GetName(), pxy.cfg.Secretkey, allowUsers)
-	if errRet != nil {
-		err = errRet
-		return
-	}
-	pxy.listeners = append(pxy.listeners, listener)
-	xl.Infof("stcp proxy custom listen success")
-
-	pxy.startCommonTCPListenersHandler()
+	err = pxy.startVisitorListener(pxy.cfg.Secretkey, pxy.cfg.AllowUsers, "stcp")
 	return
 }