session.servername
与会话关联的 SNI（服务器名称指示）主机名。在客户端 hello 处理之前，这是 null。一旦 hello 处理完成，这要么是主机名字符串，要么如果握手没有 SNI，则为 false。
🌐 The SNI (Server Name Indication) host name associated with the session. This is
null before the client hello is processed. Once the hello has been
processed, this is either the host name string or false if the handshake
had no SNI.