Use SI (cm) units instead of US (inches) ones

Packages describing “metric” as local USE flag

Package “metric” Flag Description
sci-geosciences/gmt Use SI (cm) units instead of US (inches) ones
media-gfx/jpeg2ps Default to A4 paper size
app-text/xpdf Use A4 instead of US Letter paper size by default for PostScript output. This option has effect only if libpaper USE flag is disabled
media-gfx/potrace default to a4 paper size and metric measurement

All packages providing a “metric” USE flag (3)