The Health app is not supposed to be on your watch - nor can it be added to it.
Health and fitness data from Apple Watch, iPhone and any other data sources is aggregated within the Health app on your iPhone. The app presents data in charts that take account of all data sources, without double counting any data that may be provided by multiple sources for the same time period (such as steps data, when you are both wearing your watch and carrying your iPhone).
Use the Health app on your iPhone or iPod touch - Apple Support
The Heartbeat Glance (which is used to manually measure your heart rate on your watch) is not an app, so there is not supposed to be - and nor is there - an icon for it on the Home screen on your watch, where icons for installed apps are shown. (The Home screen is accessed by pressing once on the Digital Crown).
Instead, when viewing the time on your watch face: swipe up on the screen, then swipe left / right until you find the Heartbeat Glance. Swipe down on the screen to exit.