The best heart rate monitor you can get is the Polar H10.

It's the most accurate heart rate monitor for consumers.

And it's absolutely open to anything, they even have an open source SDK to build your own apps: https://github.com/polarofficial/polar-ble-sdk

Very rare in a sea of close sourced crap.

And if you want an app to use it with, I can recommend Runnerup: https://github.com/jonasoreland/runnerup

An amazing open source app for logging your runs, with GPS, audio, and heart rate.