func WithHTTPClient(client *http.Client) ClientOption {
	return func(metaclient *Client) { metaclient.client = client }
}
WithHTTPClient makes the metadata client use the given HTTP client.
package metadata
import (
	"bufio"
	"encoding/json"
	"fmt"
	"io"
	"io/ioutil"
	"net/http"
	"net/url"
	"path"
	"time"
)
const (
	maxErrMsgLen = 128  
	defaultTimeout = 2 * time.Second
	defaultPath    = "/metadata/v1/"
)
var (
	defaultBaseURL = func() *url.URL {
		u, err := url.Parse("http://169.254.169.254")
		if err != nil {
			panic(err)
		}
		return u
	}()
)
type ClientOption func(*Client)