Replying to my own issue:

First I misunderstood module registration:

#include <logging/log.h>
// Must be done outside the function
LOG_MODULE_REGISTER(toto, LOG_LEVEL_DBG);

void main(void)
{
    //...
    LOG_INF("Hello World ! %s\n", CONFIG_BOARD);
    // ...
}

Then, I used west build -t menuconfig to enable:

[*] Logging-->    // This one was not enabled
    [*] Enable Serial Wire Output (SWO) backend
    (0)     Set SWO output frequency 

After that I managed to get traces via the JLINK SWO Viewer.

However, it only works if I launch the SWO viewer AFTER my application is launched. If I trigger a reset of the target during a swoview session, I lost traces. I have to re-launch the swo viewer to get the logs again, meaning that I cannot have traces of a reboot for example and I cannot figure out why because the file log_backend_swo_init has the procedure described in JLINK User guide (3.8.4 Configure SWO output after device reset)