promql/value.go
func (r *Result) Scalar() (Scalar, error) {
	if r.Err != nil {
		return Scalar{}, r.Err
	}
	v, ok := r.Value.(Scalar)
	if !ok {
		return Scalar{}, errors.New("query result is not a Scalar")
	}
	return v, nil
}
func (r *Result) Scalar() (Scalar, error) {
	if r.Err != nil {
		return Scalar{}, r.Err
	}
	v, ok := r.Value.(Scalar)
	if !ok {
		return Scalar{}, errors.New("query result is not a Scalar")
	}
	return v, nil
}
// Scalar returns a Scalar value. An error is returned if
// the result was an error or the result value is not a Scalar.