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: meta.go
    path of file: ./repos/gomplate/gcp
    
    the code of the file until where you have to start completion: package gcp

import (
	"context"
	"fmt"
	"io"
	"net/http"
	"strconv"
	"strings"
	"sync"
	"time"

	"github.com/hairyhenderson/gomplate/v4/env"
)

// DefaultEndpoint is the DNS name for the default GCP compute instance metadata service.
var DefaultEndpoint = "http://metadata.google.internal"

var (
	// co is a ClientOptions populated from the environment.
	co ClientOptions
	// coInit ensures that `co` is only set once.
	coInit sync.Once
)

// ClientOptions contains various user-specifiable options for a MetaClient.
type ClientOptions struct {
	Timeout time.Duration
}

// GetClientOptions - Centralised reading of GCP_TIMEOUT
// ... but cannot use in vault/auth.go as differ
    
func (c *MetaClient) retrieveMetadata(ctx context.Context, url string, def ...string) (string, error) {
	if value, ok := c.cache[url]; ok {
		return value, nil
	}

	if c.client == nil {
		timeout := c.options.Timeout
		if timeout == 0 {
			timeout = 500 * time.Millisecond
		}
		c.client = &http.Client{Timeout: timeout}
	}

	request, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
	if err != nil {
		return returnDefault(def), nil
	}
	request.Header.Add("Metadata-Flavor", "Google")

	resp, err := c.client.Do(request)
	if err != nil {
		return returnDefault(def), nil
	}

	defer resp.Body.Close()
	if resp.StatusCode > 399 {
		return returnDefault(def), nil
	}

	body, err := io.ReadAll(resp.Body)
	if err != nil {
		return "", fmt.Errorf("failed to read response body from %s: %w", url, err)
	}
	value := strings.TrimSpace(string(body))
	c.cache[url] = value

	return value, nil
}