[metapost] uright greek "mu" symbol in Metapost

[Manuel Alexandre Pouchon]

Hi,

I am trying to have a label with microns in it (µm).
I know that in LaTeX it is a problem  to print upright "mu", because the 
mathematical environment $..$ will only provide the italic greek 
symbols. However, for LaTeX there are packages to handle this problem.
How can I manage this problem in the MetaPost labels? In the btex ... 
etex environment only TeX is allowed (?). And in any case, how could I 
introduce any packages in the btex ... etex environment.
I also tried to access the ASCII symbol 181 directly using char, but 
MetaPost only returns something like "^^b5" instead of "mu".

I would really appreciate, if any could help me solving this problem.

Cheers
Manuel