func FileListerFunc(bh *gcs.BucketHandle, prefix string, filter *regexp.Regexp) FileLister {
	return func(ctx context.Context) ([]*storage.ObjectAttrs, int64, error) {
		return bh.GetFilesSince(ctx, prefix, filter, time.Time{})
	}
}
FileListerFunc creates a function that returns a slice of *storage.ObjectAttrs.
On certain GCS errors, it may return partial result and an error.
TODO - consider moving this to GardenerAPI.
package active
import (
	"context"
	"log"
	"regexp"
	"sync"
	"time"
	"google.golang.org/api/iterator"
	"cloud.google.com/go/storage"
	"github.com/m-lab/etl-gardener/tracker"
	"github.com/m-lab/etl/etl"
	"github.com/m-lab/etl/metrics"
	"github.com/m-lab/go/cloud/gcs"
	"github.com/m-lab/go/logx"
)
var debug = logx.Debug
type FileLister func(ctx context.Context) ([]*storage.ObjectAttrs, int64, error)