We have a way to fetch the number sessions unique per device and the number of The New Feature uses, this can be done with a public API and requires implementation of two events to be sent by mobile applications to Google Analytics server. It will give us a statistics of the sessions when The New Feature was used, although it doesn't directly reflect individual users activity.
Ex: the app was opened 1000 times among all unique users (devices), The New Feature has been opened 200 times, the resulting value is 200/1000 or 20%. The drawback is that at this particular case we have no way to tell that is wasn't one user who has opened The New Feature 199 times and another one who has opened it just once, the real retention rate is low to none.
The secondary statistics that we are aiming to be able to calculate is the percentage of unique users who have used The New Feature at least N times during the given period. This statistics should be a closer representation of the real The New Feature retention as it will both show the share of users who were using the feature and the dynamics of frequency. For that we are not clear of which events are needed to be set up.
Ex: the app was opened 1000 times: user A used The New Feature 10 times, user B 5 times, user C 4 times, most of the other users who used The New Feature opened it 2 times - The New Feature was opened 200 times in total. The resulting percentage of users: 10% have opened The New Feature at least once, 8% used it at least 2 times, ..., 1% used it at least 10 times.
The numbers from the second example are giving us more useful information about how often the new feature is being used, but it isn't clear how we can set it up. We would need a kind of the event that shows a number of uses of The New Feature unique by the users (not just sessions) and I think the event values might be used to distinguish the users, will it be possible to get the number of unique users who has triggered the event at least N times this way ? Any other suggestion is welcome.