I think this stuff would work great. I used it on one wrench that I made (the top one with the round handle), as a rust preventive coat but I think it would work to mark wrenches too.

It is quick drying & cheap. Seems to be very similar to the permanent marking pens. One can of blue for all the metrics would probably do it. It is also available in red for those who want to mark both.

Here's a link to one place that sells it:

http://www.use-enco.com/CGI/INPDFF?PMPAGE=290&PARTPG=INLMK3&PMITEM=505-1393