Exclude Stories from WIP Limit

Actual Behavior:

• In the Agile board configuration, Column management, I can select a column constraint from the following options:
  1. None
  2. Issue Count
  3. Issue Count, excluding sub-tasks

Expected Behavior:

• There should be fourth option "Only sub-tasks"

Context

Our main working items after Planning 2 are sub-tasks we have created with the whole team. Each sub-tasks should be a maximum of 1 day effort. I as a scrum master want to restrict the number of open sub-tasks using the WIP limit. The problem is, that also the stories will count in. To work around that, we keep the story OPEN until we have resolved all sub-tasks an then also resolve the Story. That has the disadvantage that we cannot calculate cycle time for stories.