I run a couple of times a week, and while using the built-in training app, I sometimes use an app called Intervals Pro, which has an Apple Watch app. When I use intervals, I can check the time / distance of the clock in the middle of training and see a heart icon with my current heart rate. So I guess it tracks my heart rate.
After my interval workout, I can look in the Activity app and see the average heart rate in the section for average heart rate shows average heart rate. But the heart rate map is unavailable.
When I use the built-in training app on the Watch, my average heart rate is blank in addition to a blank chart.