Get all fieldnames of index from

[aniket_mandhare (Makrand Mandhare)]

How to get all only fieldnames(key name) and not the values of it from index in Elasticsearch
I tried using following request

GET /my_index/_field_caps?fields=*&filter_path=fields.*

Expected output
{
fields:{
"field_1",
"field_2.subfield_1",
"field_2.subfield_2",
"field_3.subfield_1.another_field"
}
}