Why is trace emitted twice?

https://play.golang.org/p/89gZ-Izsp_J

Emits three lines:

1. {"fields":{"app":"myapp","env":"prod"},"level":"info","timestamp":"2009-11-10T23:00:00Z","message":"heavy operation"}
2. Sleeping for 81ms
3. {"fields":{"app":"myapp","duration":81,"env":"prod"},"level":"info","timestamp":"2009-11-10T23:00:00.081Z","message":"heavy operation"}

I didn't expect line number one! It doesn't have any value, does it?