promql/engine.go
func contextDone(ctx context.Context, env string) error {
	if err := ctx.Err(); err != nil {
		return contextErr(err, env)
	}
	return nil
}
func contextDone(ctx context.Context, env string) error {
	if err := ctx.Err(); err != nil {
		return contextErr(err, env)
	}
	return nil
}
// contextDone returns an error if the context was canceled or timed out.