As for the metrics to rely on, my response is simply stupid — it depends. It really depends on your business goals and the type of product you build. At the same time metrics like user sign-ups, reviews, and direct feedback are always actionable
— regardless of what you choose, it should be comparative and understandable.
One more thing I should mention. Remember, it's just an MVP, so high numbers would happen somewhere else.
If the idea fails (poor outcome plus zero or bad feedback), junk it. Or try pivot. Actually, there's almost always something you can do until you succeed. And of course, as long as you have money in the bank:-)