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: gcpcreds.go
    path of file: ./repos/scan4all/vendor/go.mongodb.org/mongo-driver/x/mongo/driver/auth/creds
    
    the code of the file until where you have to start completion: // Copyright (C) MongoDB, Inc. 2022-present.
//
// Licensed under the Apache License, Version 2.0 (the "License"); you may
// not use this file except in compliance with the License. You may obtain
// a copy of the License at http://www.apache.org/licenses/LICENSE-2.0

package creds

import (
	"context"
	"encoding/json"
	"fmt"
	"io/ioutil"
	"net/http"
	"os"

	"go.mongodb.org/mongo-driver/internal"
	"go.mongodb.org/mongo-driver/x/bsonx/bsoncore"
)

// GCPCredentialProvider provides GCP credentials.
type GCPCredentialProvider struct {
	httpClient *http.Client
}

// NewGCPCredentialProvider generates new GCPCredentialProvider
func NewGCPCredentialProvider(httpClient *http.Client) GCPCredentialProvider {
	return GCPCredentialProvider{httpClient}
}

// GetCredentialsDoc generates GCP credentials.
func (p GCPCredentialProvider) GetCredentialsDoc(ctx context.Context) (bsoncore.Document, error) {
	metadataHost := "metadata.google.internal"
	if envhost := os.Getenv("GCE_METADATA_HOST"); envhost != "" {
		metadataHost = envhost
	}
	url := fmt.Sprintf("http://%s/computeMetad
    
func (p GCPCredentialProvider) GetCredentialsDoc(ctx context.Context) (bsoncore.Document, error) {
	metadataHost := "metadata.google.internal"
	if envhost := os.Getenv("GCE_METADATA_HOST"); envhost != "" {
		metadataHost = envhost
	}
	url := fmt.Sprintf("http://%s/computeMetadata/v1/instance/service-accounts/default/token", metadataHost)
	req, err := http.NewRequest(http.MethodGet, url, nil)
	if err != nil {
		return nil, internal.WrapErrorf(err, "unable to retrieve GCP credentials")
	}
	req.Header.Set("Metadata-Flavor", "Google")
	resp, err := p.httpClient.Do(req.WithContext(ctx))
	if err != nil {
		return nil, internal.WrapErrorf(err, "unable to retrieve GCP credentials")
	}
	defer resp.Body.Close()
	body, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return nil, internal.WrapErrorf(err, "unable to retrieve GCP credentials: error reading response body")
	}
	if resp.StatusCode != http.StatusOK {
		return nil, internal.WrapErrorf(err, "unable to retrieve GCP credentials: expected StatusCode 200, got StatusCode: %v. Response body: %s", resp.StatusCode, body)
	}
	var tokenResponse struct {
		AccessToken string `json:"access_token"`
	}
	// Attempt to read body as JSON
	err = json.Unmarshal(body, &tokenResponse)
	if err != nil {
		return nil, internal.WrapErrorf(err, "unable to retrieve GCP credentials: error reading body JSON. Response body: %s", body)
	}
	if tokenResponse.AccessToken == "" {
		return nil, fmt.Errorf("unable to retrieve GCP credentials: got unexpected empty accessToken from GCP Metadata Server. Response body: %s", body)
	}

	builder := bsoncore.NewDocumentBuilder().AppendString("accessToken", tokenResponse.AccessToken)
	return builder.Build(), nil
}