The MetricsCount key is documented in Adobe Tech Note 5012 The Type 42 Font Format Specification. According to the specification it can have 3 possible values, 0, 2 or 4.
Section 5.7 on page 22 of the document:
When a key /MetricsCount is found in a CIDFont with CIDFontType 2, it
must be an integer with values 0, 2, or 4.
To me this suggests that the MetricsCount key/value pair is optional, and as I said other interpreters don't insist on its presence. I can't possibly tell you why Adobe Distiller appears to insist on it, I don't have any experience of the internals of the Distiller PostScript interpreter. I'd have to guess that all Adobe PostScript interpreters have this 'feature' though, presumably your printer is using an Adobe PostScript interpreter.
Simply adding the MetricsCount key does not work. Why didn't you try this yourself instead of asking me ? It would have been quicker....
The error is subtly different, I suspect the answer is that your CIDFont is missing something (or has something present) which is causing Distiller to look for a MetricsCount. I can't see anything obvious in the PostScript information, so perhaps there's something in the sfnts, though that would be surprising.
Interestingly I have in front of me a PostScript file containing a CIDFont which does not have a MetricsCount entry, and which Distiller processes without a complaint.
I can't let you have the file I'm using, its a customer file. However the fact that such a file exists indicates that other such files must exist. The one I'm looking at was created by QuarkXpress. I'd suggest that you try and find a working file to compare against. I'd also suggest that you try and make a smaller, simpler, CIDFont. One with a single glyph would be favourite I'd think.