How can I use fixed-length multipart request with FormData?

Discussed in https://github.com/nodejs/undici/discussions/1420

^{Originally posted by nwtgck May 6, 2022}
Hi, I'd like to request a fixed-length multipart request. The code below requested with transfer-encoding: chunked without content-length. I assume the length of formData is statically decided.

const undici = require("undici");

(async () => {
  const formData = new undici.FormData();
  formData.append("myname", "myvalue");
  const res = await undici.request("http://localhost:8181", {
    body: formData,
  });
})();

$ nc -lp 8181
PUT / HTTP/1.1
host: localhost:8181
connection: keep-alive
content-type: multipart/form-data; boundary=----formdata-undici-0.5140957512643098
transfer-encoding: chunked

64
------formdata-undici-0.5140957512643098
Content-Disposition: form-data; name="myname"

myvalue

2a
------formdata-undici-0.5140957512643098--
0
```</div>

The entry points shown are undici.FormData and undici.request; start by tracing how FormData is serialized and how request selects transfer encoding. Compare the result with the provided netcat output, and consider the work complete when a fixed-length multipart request sends Content-Length instead of transfer-encoding: chunked.