Google hasn't announced this yet -- and, as we noted above, this is entirely opt-in, unlike its recently announced Terms of Service change that is opt-out. Starting on Nov. 11, Google's new TOS allows the company to use your name and profile photo in shared endorsements that appear in ads.
"Mobile Meter" uses Android and iOS apps to intelligently monitor web browsing habits and app usage and send the data back to Google. Why would anyone want to opt into this? Simply stated, because Google will reward them in some still-to-be-announced way for doing so.
Google, naturally, won't comment, since it never does so on what it considers "rumor and speculation." Due to that fact, it's unclear when this will go into effect and which apps the Internet giant will use for the opt-in monitoring.
Sans a reward, Google already passively collects data to improve its services. For example, Google Maps regularly sends back location metrics to enhance the service.
Google also isn't the first service to reward users for app usage data. Nielsen has already been conducting research into mobile with an Android app, promising up to $50 annually as a perk.