k6 v0.38.3 is a patch release containing a single fix
Threshold over already defined sub-metrics will result in an error (#2538)
There was a bug where we were checking if a submetric had already been added. Unfortunately, we didn't check that this will work with the one submetric we have by default http_req_duration{expected_response:true}. After v0.38.0 defining a threshold on it would result in an error.
As this definitely shouldn't happen in that case and we don't see a particular case where that will be problematic - adding a submetric again just reuses the already added one instead.
This issue has been addressed in #2539, and k6 v0.38.3 will now lead you add a threshold on http_req_duration{expected_response:true}.