Unexpected http/net response content length in Golang

Question

I am making a HTTP Get request using the code below. I proxy it through Fiddler to analyse the request. The request seems to be being made correctly, with the expected response. However, the resp.ContentLength property of the response in GO is -1 every time despite the fact that the response in Fiddler shows a positive integer value of Content-Length: 1830.

Why is the ContentLength not being picked up in GO?

Go Code

package main

import "net/http"
import "os"
import "fmt"

func main() {
    os.Setenv("HTTP_PROXY", "http://127.0.0.1:8888") // For Fiddler debugging
    resp,err := http.Get("http://www.google.com/robots.txt")
    if (err == nil) {
        if (resp.StatusCode == 200) {
            fmt.Println(resp.ContentLength) // Prints -1
        }
    }
}

Resulting Request in Fiddler

GET /robots.txt HTTP/1.1
Host: www.google.com
User-Agent: Go-http-client/1.1
Accept-Encoding: gzip

Response Received in Fiddler

HTTP/1.1 200 OK
Accept-Ranges: bytes
Vary: Accept-Encoding
Content-Type: text/plain
Content-Length: 6907
Date: Mon, 05 Mar 2018 13:53:00 GMT
Expires: Mon, 05 Mar 2018 13:53:00 GMT
Cache-Control: private, max-age=0
Last-Modified: Thu, 01 Mar 2018 18:00:00 GMT
X-Content-Type-Options: nosniff
Server: sffe
X-XSS-Protection: 1; mode=block

Answer 1

The value -1 indicates that the length is unknown. (in most cases this indicates chunked encoding).