Heart rate on watch series 5

In the watch app on the phone tap on workout then tap on workout view turn on multiple metric and then tap on outdoor run,tap edit in the top right corner and select which metrics you want see on the workout screen.

Read here: Adjust the workout settings on Apple Watch - Apple Support

Check to see if you have power saving mode turned on;

1. On your iPhone, open the Apple Watch app. 
2. Tap the My Watch tab, then tap Workout.
3. Turn Power Saving Mode off. 

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