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: checker.go
    path of file: ./repos/athens/pkg/storage/gcp
    
    the code of the file until where you have to start completion: package gcp

import (
	"context"

	"cloud.google.com/go/storage"
	"github.com/gomods/athens/pkg/config"
	"github.com/gomods/athens/pkg/errors"
	"github.com/gomods/athens/pkg/observ"
	"google.golang.org/api/iterator"
)

// Exists implements the (./pkg/storage).Checker interface
// returning true if the module at version exists in storage.
func (s *Storage) Exists(ctx context.Context, module, version string) (bool, error) {
	const op errors.Op = "gcp.Exists"
	ctx, span := observ.StartSpan(ctx, op.String())
	defer span.End()

	it := s.bucket.Objects(ctx, &storage.Query{Prefix: config.PackageVersionedName(module, version, "")})
	var count int
	for {
		attrs, err := it.Next()
		if errors.IsErr(err, iterator.
    
func (s *Storage) Exists(ctx context.Context, module, version string) (bool, error) {
	const op errors.Op = "gcp.Exists"
	ctx, span := observ.StartSpan(ctx, op.String())
	defer span.End()

	it := s.bucket.Objects(ctx, &storage.Query{Prefix: config.PackageVersionedName(module, version, "")})
	var count int
	for {
		attrs, err := it.Next()
		if errors.IsErr(err, iterator.Done) {
			break
		}
		if err != nil {
			return false, errors.E(op, err, errors.M(module), errors.V(version))
		}
		switch attrs.Name {
		case config.PackageVersionedName(module, version, "info"):
			count++
		case config.PackageVersionedName(module, version, "mod"):
			count++
		case config.PackageVersionedName(module, version, "zip"):
			count++
		}
	}

	return count == 3, nil
}