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: backend.go
    path of file: ./repos/goldfish/vendor/github.com/hashicorp/vault-plugin-auth-gcp/plugin
    
    the code of the file until where you have to start completion: package gcpauth

import (
	"context"
	"fmt"
	"net/http"
	"runtime"
	"sync"

	"github.com/hashicorp/go-cleanhttp"
	"github.com/hashi
    
func (b *GcpAuthBackend) initClients(s logical.Storage) (err error) {
	config, err := b.config(s)
	if err != nil {
		return err
	}

	var httpClient *http.Client
	if config == nil || config.Credentials == nil {
		// Use Application Default Credentials
		ctx := context.WithValue(context.Background(), oauth2.HTTPClient, cleanhttp.DefaultClient())

		httpClient, err = google.DefaultClient(ctx, b.oauthScopes...)
		if err != nil {
			return fmt.Errorf("credentials were not configured and fallback to application default credentials failed: %v", err)
		}
	} else {
		httpClient, err = util.GetHttpClient(config.Credentials, b.oauthScopes...)
		if err != nil {
			return err
		}
	}

	userAgentStr := fmt.Sprintf("(%s %s) Vault/%s", runtime.GOOS, runtime.GOARCH, version.GetVersion().FullVersionNumber(true))

	b.iamClient, err = iam.New(httpClient)
	if err != nil {
		b.Close()
		return err
	}
	b.iamClient.UserAgent = userAgentStr

	b.gceClient, err = compute.New(httpClient)
	if err != nil {
		b.Close()
		return err
	}
	b.gceClient.UserAgent = userAgentStr

	return nil
}