A further comment on Metrogel.

Metrogel is a specially formulated form of metronidazole, specifically intended
for vaginal use only.  It is only minimally absorbed via this route.  The
maternal plasma level is less than 2% of the level attained with one single
oral tablet.  So you can see,  maternal blood levels are miniscule, hence
breast milk levels would be minimal as well.

Tom Hale