If there isn’t a strategy with associated metrics already, this game is in a much, MUCH worse state than anyone realises. I doubt that’s the case, though.
Balance at the level we’re talking about here isn’t dictated by low-level metrics like winrates in a narrow scope, it’s dictated by the experience the devs are trying to produce for its audience (which itself is at least partially dictated by the higher-ups in the company, but that’s a whole other level).
The combination of specific features of the game is what’s used to tailor the experience. That’s where real balancing takes place. That’s why devs like it when you tell them how something feels rather than suggesting specific changes.
If I had to take a wild stab, I’d say their metrics are going to be primarily looking at things like populations and behaviours before hero statistics.