Unmarshalling null ProxyURL

[OlivierCazade]

When unmarshalling a ProxyURL field set to null, it get unmarshalled to an empty URL.

Contrary to a null pointer, an empty string will be handled by the http package like a valid Proxy, resulting in some connection error.
https://pkg.go.dev/net/http#Transport

This PR change empty ProxyURL to nul URL during the validate phase of unmarshalling.

[roidelapluie]

Is this related to json unmarshalling?

[OlivierCazade]

Yes, when marshalling to json an httpconfig and then unmarshalling it, the proxyURL move from beeing nil to an empty string.

Here is a code snippet:

package main

import (
	"encoding/json"
	"fmt"

	promConfig "github.com/prometheus/common/config"
)

func main() {
	config := promConfig.HTTPClientConfig{}
	bs, _ := json.Marshal(config)

	config2 := promConfig.HTTPClientConfig{}
	json.Unmarshal(bs, &config2)
	fmt.Println(config2)
}

In the case of the HTTPClientConfig, this change the behaviour, the proxyURL used here :

common/config/http_config.go

Line 464 in 1482587

Proxy: http.ProxyURL(cfg.ProxyURL.URL),

An empty string will be considered as a valid URL while a nil pointer will be ignored.

I first wanted to submit a patch to the URL unmarshall function, but it looked likes this was done this way intentionnaly:
558a787

• the marshalling function explicitly marshall to json
• unit tests explicitly check that a null URL is unmarshalled to an empty string

So, I assumed this was used somewhere else, and I submited this patch instead to only affect HTTPClientConfig.

[roidelapluie]

Can you add tests for this? Or maybe we need to implement marshaljson properly