Change stream events now include a 'wallTime' field with the wall time from the mongod that the change event originated from. previously this was only exposed if the showExpandedEvents change stream option was set to true.
We currently have the 'wallTime' field guarded behind the 'showExpandedEvents' flag. This was done so that we don't add this output field directly into API version 1. Given the requests from the users, we thought it would be a good idea to always expose this.