Why event! macro cannot take quoted field name?

[tisonkun]

                info!(
                    "term" = self.term,
                    "message_term" = m.term,
                    "msg type" = ?m.msg_type(),
                    "received a message with higher term from {from}", 
                    m.from);

cannot compile:

error: expected `,`, found `=`
    --> path/to/file.rs:1291:28
     |
1291 |                     "term" = self.term,
     |                            ^ expected `,`

while from the docs, span! support such syntax https://docs.rs/tracing/latest/tracing/index.html#recording-fields.