Field capabilities request is invalid

What is the bug?

Field capabilities request is serializing fields as json body of the POST request. But based on documentation and my testing, fields has to be a query parameter instead.

How can one reproduce the bug?

client.fieldCaps(r -> r.allowNoIndices(true).fields("*"))

this request should return something. Instead, it's returning following error:

{
  "type": "illegal_argument_exception",
  "reason": "specified fields can't be null or empty"
}

What is the expected behavior?

Returning response with fields without errors. When building the field cap request, the fields content has to be used as query param, not body of the POST req.

What is your host/environment?

Opensearch 2.19.3 server on linux/ubuntu