vendor/github.com/hashicorp/terraform/httpclient/useragent.go

package httpclient

import (
	"fmt"
	"log"
	"net/http"
	"os"
	"strings"

	"github.com/hashicorp/terraform/version"
)

const userAgentFormat = "Terraform/%s"
const uaEnvVar = "TF_APPEND_USER_AGENT"

func UserAgentString() string {
	ua := fmt.Sprintf(userAgentFormat, version.Version)

	if add := os.Getenv(uaEnvVar); add != "" {
		add = strings.TrimSpace(add)
		if len(add) > 0 {
			ua += " " + add
			log.Printf("[DEBUG] Using modified User-Agent: %s", ua)
		}
	}

	return ua
}

type userAgentRoundTripper struct {
	inner     http.RoundTripper
	userAgent string
}

func (rt *userAgentRoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
	if _, ok := req.Header["User-Agent"]; !ok {
		req.Header.Set("User-Agent", rt.userAgent)
	}
	return rt.inner.RoundTrip(req)
}