I have heard that HealthKit can be used to gather health-related data using the iPhone’s sensors in real-time, and I know that there are several third-party wearable biometric sensors that can interface with iOS and are supposed to be compatible with HealthKit (but it’s not clear to me whether that means they’re compatible with HealthKit itself or just the Health app). What I want to know is: does the HealthKit API provide a (relatively simple) way to access data from these external sensors in real-time as they're coming in? Or is it really only designed to do that for sensors that are already in the iOS device itself?
From a user level - the health app consumes data that is stored in the HealthKit framework. From the developer and hardware manufacturer side, HealthKit framework is an API for them to do things reading and writing the data so that what you describe is clearly possible.
Even watch OS streams live data - so that's a core feature of bluetooth low energy. Once the data is on iOS, it's a matter of the developer placing it under health kit control. This is something Apple expects and allows with their code and SDK/API.
Now, not all data can be written to all areas - for instance, Apple's workout app doesn't allow third party contributions on Watch OS 1.0 and 2.0 but it does allow workout data to be stored in the database along with the Apple watch workout app data.