Recommend System AB test metric events
This all comes down to your objectives that come to mind which you could use:

We care about the entire website.  The new recommender system will boost total payments either directly or indirectly.  We do not want the recommender to hurt total payments.
We care only about the recommender, perhaps we believe
this is the future of our business and we want to eventually replace
the entire website.  It's critical the recommender directly drive payments.

Either way, I would recommend tracking recommender-associated payments and non-associated payments.  It's important to understand direct and ecosystem effects.  It's possible the recommender is exceptionally successful at driving clicks and finding users great games, but also finding them games which do not monetize successfully.  It's also possible the recommender is so awful that it scares people away from the website.  You will want to measure and track both direct and indirect effects, but how you make your decision will depend on the objective you are trying to achieve.