Since collections are not structured i.e. all documents don't follow the same structure, it becomes hard to find out the exhaustive list of keys in a document.
Even though there are answers on Stackoverflow where they have solved it using MapReduce in MongoDB, the easiest way I found was to use the "Analyze Schema" option of MongoDB Compass. It gives an overview of all the keys and data variations in a nice UI.
Comment below if you have an easier way to do this.