The TTL monitor is generally considered as a background task. However, it is used to clean up expired sessions which has implications on memory usage due to how transactions are cleaned up. Therefore, we do not want to mark it as background by default while the effects are not well defined.

We will mark it by default as non-deprioritizable to maintain the prior behavior and will introduce a new server parameter to change it to background. This server parameter can either be used by mongotune or by atlas once verification has been done to ensure that it is safe to be background.