Google Fit app got new ways to measure heart rate and respiratory rate. The feature is still being rolling out to more Pixel users.
If you are lucky enough you may find new promo banners in the app that will guide you through the process.
For the heart rate measurement, you would need to put your finger on the rear camera. And for respiratory rate measurement, you would need to hold your device on the table and look to a front camera.
All the results will become visible on the main tab afterward.
Thanks to @saipraveenbammidi for the tip and screenshots!