Add Release notes for v0.38.3

Author: mstoykov

release notes/v0.38.3.md

@@ -0,0 +1,9 @@
+k6 v0.38.3 is a patch release containing a single fix
+
+### Threshold over already defined sub-metrics will result in an error ([#2538](https://github.com/grafana/k6/issues/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](https://github.com/grafana/k6/pull/2539), and k6 `v0.38.3` will now lead you add a threshold on `http_req_duration{expected_response:true}`.