Test if SketchUp uses metric units

metric.rb

def metric?(model = Sketchup.active_model)
  unit_options = model.options["UnitsOptions"]
  return false unless unit_options["LengthFormat"] == Length::Decimal

  [
    Length::Millimeter,
    Length::Centimeter,
    Length::Meter
  ].include?(unit_options["LengthUnit"])
end

# Example usage
default = metric? ? 100.mm : 4.inch