Its tracked but its not in the headline job numbers - ie the monthly "unemployment rate".I know you don't want to hear this but the under-employed are included in the official numbers.
https://www.bls.gov/news.release/empsit.t15.htm
So the unemployment rate is simply the percentage of people that want to work but have no job at all. The underemployed or part time employed are counted as employed in the headline number.
The published rate is a simply a survey. They contact 60,000 households - and ask - how many people in your household want to work, and how many have a job - part or full time. If you don't want to work, your not counted at all.