A few days back I got a Xiǎomǐ Mi Fit2 bracelet, and I recently discovered that it can be used with Strava on iOS to capture heart-rate data. Putting together the heart-rate data with the biking data is something I had to do manually with my Fitbit, with the Mi Fit2 it is reasonably easy.
As far as I can tell, the moment the Mi Fit2 is paired with your phone, it is recognised by Strava as a heart-rate measurement device. Maybe it implements the relevant Bluetooth LE profile. The problem is, the Mi Fit2 only measures the heart-rate when you tell it, i.e. when the heart-rate is selected on the watch.
The way to force the device to measure the heart-rate regularly is by the way of the App. The following instruction are based on Mi Fit app version 2.4.3 (algorithm 1.0.95). Go to the play menu, scroll to the experimental section, and select behaviour tagging, and then select the relevant activity (there is biking). One thing to know is to stop you have to tap and hold the pause button and after some time, the option to end appears. As long as the activity lasts, the bracelet will measure the heart-rate regularly.