Creating custom fields in indexed records of elastic search

Elastic Stack Elasticsearch
1

Hi, I have the following setup on red hat linux,

Fluentd (data collector) --> Elasticsearch (Indexing data) --> Kibana (Visualization)

Fluentd is collecting data from some resource which I want to reform. Say for example I am getting students information from fluentd like student information and marks in subjects. I want to create new fields like total marks, percentage and grade along with existing fields.

I will be using these fields in kibana to create graphs.

Where and how to create these custom fields?
Where should calculation of these fields should happen?

2

You can do aggregations on the data to get things like total marks, but they aren't added as fields, it's all dynamic in KB.