http: Only append to xfh if it was not the last value appended (#26417)

Signed-off-by: River phillips <riverphillips1@gmail.com>

Mirrored from https://github.com/envoyproxy/envoy @ 6aa3fa96240b79655652cce4918a7a14867a531b

envoy/config/route/v3/route_components.proto

@@ -1188,7 +1188,8 @@ message RouteAction {
   // :ref:`host_rewrite_header <envoy_v3_api_field_config.route.v3.RouteAction.host_rewrite_header>`, or
   // :ref:`host_rewrite_path_regex <envoy_v3_api_field_config.route.v3.RouteAction.host_rewrite_path_regex>`)
   // causes the original value of the host header, if any, to be appended to the
-  // :ref:`config_http_conn_man_headers_x-forwarded-host` HTTP header.
+  // :ref:`config_http_conn_man_headers_x-forwarded-host` HTTP header if it is different to the last value appended.
+  // This can be disabled by setting the runtime guard `envoy_reloadable_features_append_xfh_idempotent` to false.
   bool append_x_forwarded_host = 38;
 
   // Specifies the upstream timeout for the route. If not specified, the default is 15s. This