Where should i store my google service account keys?

good day :D this question might sound pretty noob but i really needed to ask, where is a good place i can store my google service account keys (or any other important private files like these)?

I am implementing Core Reporting API on getting data from Google Analytics. I have already created a service account and downloaded the .p12 key, and i can access my stuff already. But I'd really want to ask you people on where is the proper location for files like these(.p12 key).

For additional information, i'm creating a website using yii2 framework and linux.

Yes, i know you're thinking i dont know the basic stuff like security but thats why im asking so i can learn, thanks in advance for any advice you'll write.