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InfluxDB
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Alright, that sounds plausible. One similar example of which I know is the Phonograph music player. Its free and open source on GitHub. It has in-app purchases that basically unlocks extra cosmetics like additional themes and looks of the app. You can check their source code on how they do it, I guess it would be quite similar to what you would want to achieve. There's also Retro Music which is a fork of Phonograph, same thing with in-app purchases to unlock more cosmetics.
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.