FitnessSyncer is huge “fitness cloud integration” platform, it is free for generic use. What would be great to integrate with them and grab almost every possible health metric (activities, steps, heartrate, weight and much more).
FitnessSyncer holds database of all synced metrics, so it acts as Google.Fit nowadays. Users even able to create activity metrics by hand (in contrast to syncing from other fitness clouds).
Their site says “please contact us about an API” so I guess this is theoretically possible.