How to work on object to get 0,1,2,3 out of it

Elastic Stack Logstash
1

"eventInfo": {
"delete.user.0.firstname": "david",
"delete.user.0.lastname": "flank",
"delete.user.0.city": "austin",
"delete.user.1.firstname": "martin",
"delete.user.1.lastname": "king",
"delete.user.1.city": "dallas",
"delete.user.2.firstname": "Samantha",
"delete.user.2.lastname": "philip",
"delete.user.2.city": "motogomery",
"delete.user.3.firstname": "john",
"delete.user.3.lastname": "david",
"delete.user.3.city": "hilton",

2

You could do that in a ruby filter. This post gives an example, although obviously it will require changes to the pattern and capture group references.