[NTG-context] How to add margin around standalone MetaPost?

[Vincent Hennebert]

Take the following MetaPost file:

beginfig(0)
  ahlength := 20;
  linejoin := mitered;
  drawarrow origin--(40,0) withpen pencircle scaled 5;
endfig;

Render it into PDF with ConTeXt. Witness the cut out ends of the arrow.
How to fix that?

Being able to add some margin would be a solution, which may be
desirable anyway in some cases. Is that possible?

Thanks,
Vincent