How to insure myself against unexpected firebase costs?

I have almost completed building my MVP using Firebase as the backend. However, I'm extremely wary of incurring unexpected costs (I'm pretty new to programming, so there might be some bug that I missed. There could also be DDOS attacks which I assume would cause my costs to go up exponentially)
So how can I insure myself against this?
I'm thinking of following this approach:
Use a credit card that has a very low limit on it for payment. My only concern here is that Google might take legal action against me if they can't collect payment for the service already provided (Afaik, they charge you at the end of each month). Someone, please let me know if that's possible.