How can users be incentivized to opt-in, particularly with a free-to-use application? I can see a case for ad-supported software, a developer could reduce or eliminate ads for that user in exchange for telemetry data...
Ask nicely. After all, you want them to do you a favor.
If you're less into respect and more into manipulation, offer them a meaningless trinket. A sticker on the app's home screen saying "I helped", or something.
