We could help you better if you provide real documents. We can work with just a schema like page_id|event_type but then we need to create documents in our env. rather than just cut-n-pasting what you provide. But time being in short supply we sometimes just skip over or we supply untested solution.
As a first step, you should have a single _id field in your $group. Do for event_type what you did for page_id. You will end up with documents out of the first stage like:
Hi, Thanks for the response! What you posted is correct, I am also getting similar result, but I want project the data in single row as i mentioned in question
I could not use the sample 2 documents you supplied as they where in a quote block rather than a pre, code or triple back ticks. The quotes were all screwed up and I had to type the documents so I used short field names.