Can someone please give me a script that changes the ruler metric?

Hello,

Try following snippet

var units = 3; //(0 to 6)
app.preferences.setIntegerPreference("rulerType", units);

/*Unit list
0 : point
1 : pica
2 : inch
3 : mm
4 : cm
5 : H/Q
6 : px */

This will change the ruler units in Units for "General" in Preferences -> Units. So, you can combine this with solution of femkeblanco 

You can set the units when you create a new document:

var preset1 = new DocumentPreset;
preset1.units = RulerUnits.Inches;
var doc1 = app.documents.addDocument("Basic CMYK", preset1);

but I don't think you can change them after that.

I actually couldn't find a way to do this with a script, but hopefully someone else will know. But just checking that you know you can right-click on the ruler to change units?