Use different user-agent for twitter links (#16210)

app/post_metadata.go

@@ -410,6 +410,12 @@ func (a *App) getLinkMetadata(requestURL string, timestamp int64, isNewPost bool
 
 		client := a.HTTPService().MakeClient(false)
 		client.Timeout = time.Duration(*a.Config().ExperimentalSettings.LinkMetadataTimeoutMilliseconds) * time.Millisecond
+		mmTransport := a.HTTPService().MakeTransport(false)
+		client.Transport = mmTransport.Transport
+
+		if strings.HasPrefix(requestURL, "https://twitter.com/") || strings.HasPrefix(requestURL, "https://mobile.twitter.com/") {
+			request.Header.Add("User-Agent", "facebookexternalhit/1.1 (+http://www.facebook.com/externalhit_uatext.php)")
+		}
 
 		var res *http.Response
 		res, err = client.Do(request)

services/httpservice/client.go

@@ -136,7 +136,7 @@ func dialContextFilter(dial DialContextFunction, allowHost func(host string) boo
 	}
 }
 
-func NewTransport(enableInsecureConnections bool, allowHost func(host string) bool, allowIP func(ip net.IP) bool) http.RoundTripper {
+func NewTransport(enableInsecureConnections bool, allowHost func(host string) bool, allowIP func(ip net.IP) bool) *MattermostTransport {
 	dialContext := (&net.Dialer{
 		Timeout:   ConnectTimeout,
 		KeepAlive: 30 * time.Second,

services/httpservice/httpservice.go

@@ -25,7 +25,7 @@ type HTTPService interface {
 	// - A timeout for end-to-end requests
 	// - A Mattermost-specific user agent header
 	// - Additional security for untrusted and insecure connections
-	MakeTransport(trustURLs bool) http.RoundTripper
+	MakeTransport(trustURLs bool) *MattermostTransport
 }
 
 type HTTPServiceImpl struct {
@@ -52,7 +52,7 @@ func (h *HTTPServiceImpl) MakeClient(trustURLs bool) *http.Client {
 	}
 }
 
-func (h *HTTPServiceImpl) MakeTransport(trustURLs bool) http.RoundTripper {
+func (h *HTTPServiceImpl) MakeTransport(trustURLs bool) *MattermostTransport {
 	insecure := h.configService.Config().ServiceSettings.EnableInsecureOutgoingConnections != nil && *h.configService.Config().ServiceSettings.EnableInsecureOutgoingConnections
 
 	if trustURLs {