HRM is requested it searches on Bluetooth for a heart rate monitor, connects, and sends data back using the `Bangle.on('HRM')` event as if it came from the on board monitor.
The characteristics of your selected sensor are cached in the settings. That means if your sensor changes, e.g. by firmware updates or similar, you will need to re-scan in the settings to update the cache of characteristics. This is done to take some complexity (and time) out of the boot process.
Scanning in the settings will do 10 retries and then give up on adding the sensor. Usually that works fine, if it does not for you just try multiple times. Currently saved sensor information is only replaced on a successful pairing. There are additional options in the Debug entry of the menu that can help with specific sensor oddities. Bonding and active scanning can help with connecting, but can also prevent some sensors from working. The "Grace Periods" just add some additional time at certain steps in the connection process which can help with stability or reconnect speed of some finicky sensors. Defaults should be fine for most.
* Off - Internal HRM is used, no attempt on connecting to BT HRM.
* Default - Replaces internal HRM with BT HRM and falls back to internal HRM if no valid measurements received.
* Both - The BT HRM needs to be started explicitly by an app that wants to use it. BT HRM has its own event and is completely separated from the internal HRM. Apps not supporting the BT HRM will not see the BT HRM measurements.
* Custom - Combine low level settings as you see fit.
The recorder plugin can record the BT HRM event (blue) and the original unchanged HRM event (green). This is mainly useful for debugging purposes or comparing the BT with the internal HRM, as the resulting "merged" HRM can be recorded using the default HRM recorder.
