You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: discovery.go
    path of file: ./repos/juju/provider/azure/internal/azureauth
    
    the code of the file until where you have to start completion: // Copyright 2016 Canonical Ltd.
// Licensed under the AGPLv3, see LICENCE file for details.

package azureauth

import (
	"context"
	"net/http"
	"net/url"
	"regexp"
	"strings"

	"github.com/Azure/azure-sdk-for-go/sdk/azcore"
	"github.com/Azure/azure-sdk-for-go/sdk/azcore/arm"
	"github.com/Azure/azure-sdk-for-go/sdk/azcore/policy"
	"github.com/Azure/azure-sdk-for-go/sdk/resourcemanager/resources/armsubscriptions"
	"github.com/juju/errors"
	"github.com/juju/utils/v3"

	"github.com/juju/juju/provider/azure/internal/errorutils"
)

const authenticateHeaderKey = "WWW-Authenticate"

// recordAuthHeaderPolicy records the value of a http response auth header.
type recordAuthHeaderPolicy struct {
	authHeader string
}

func (p *recordAuthHeaderPolicy) Do(req *policy.Request) (*http.Response, error) {
	resp, err := req.Next()
	
    
func DiscoverTenantID(ctx context.Context, subscriptionID string, clientOpts arm.ClientOptions) (string, error) {
	// We make an unauthenticated request to the Azure API, which
	// responds with the authentication URL with the tenant ID in it.

	authRecorder := &recordAuthHeaderPolicy{}
	clientOpts.PerCallPolicies = append(clientOpts.PerCallPolicies, authRecorder)

	client, err := armsubscriptions.NewClient(&fakeCredential{}, &clientOpts)
	if err != nil {
		return "", errors.Trace(err)
	}

	_, err = client.Get(ctx, subscriptionID, nil)
	if err == nil {
		return "", errors.New("expected unauthorized error response")
	}
	if code := errorutils.StatusCode(err); code != http.StatusUnauthorized {
		return "", errors.Annotatef(err, "expected unauthorized error response, got %v", code)
	}

	header := authRecorder.authHeader
	if header == "" {
		return "", errors.Errorf("%s header not found", authenticateHeaderKey)
	}
	match := authorizationUriRegexp.FindStringSubmatch(header)
	if match == nil {
		return "", errors.Errorf(
			"authorization_uri not found in %s header (%q)",
			authenticateHeaderKey, header,
		)
	}

	authURL, err := url.Parse(match[1])
	if err != nil {
		return "", errors.Annotatef(err, "cannot parse auth URL %q", match[1])
	}

	// Get the tenant ID portion of the auth URL.
	path := strings.TrimPrefix(authURL.Path, "/")
	if _, err := utils.UUIDFromString(path); err != nil {
		return "", errors.NotValidf("authorization_uri %q", authURL)
	}
	return path, nil
}