Heart rate
Not showing highlighted in metrics in workouts
Apple Watch
It may help to check the following settings:
On your iPhone, go to: Settings > Privacy > Motion & Fitness - check that both Fitness Tracking and Health are enabled.
On your iPhone, in the Watch app, go to: My Watch > Privacy > Motion & Fitness - check that both Fitness Tracking and Heart Rate are enabled.
On your iPhone, in the Watch app, go to: My Watch > General > turn off Workout Power Saving Mode.