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: lister.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"
	"strings"

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

// List implements the (./pkg/storage).Lister interface.
// It returns a list of versions, if any, for a given module.
func (s *Storage) List(ctx context.Context, module string) ([]string, error) {
	const op errors.Op = "gcp.List"
	ctx, span := observ.StartSpan(ctx, op.String())
	defer span.End()

	modulePrefix := strings.TrimS
    
func (s *Storage) List(ctx context.Context, module string) ([]string, error) {
	const op errors.Op = "gcp.List"
	ctx, span := observ.StartSpan(ctx, op.String())
	defer span.End()

	modulePrefix := strings.TrimSuffix(module, "/") + "/@v"
	it := s.bucket.Objects(ctx, &storage.Query{Prefix: modulePrefix})
	paths := []string{}
	for {
		attrs, err := it.Next()
		if errors.IsErr(err, iterator.Done) {
			break
		}
		if err != nil {
			return nil, errors.E(op, err, errors.M(module))
		}
		paths = append(paths, attrs.Name)
	}

	return extractVersions(paths), nil
}