Giardia is one of those "tag alongs". Giardia is a protozoa. Metronidazole is an antibiotic and isn't effective against Giardia. I suspect the Giardia is secondary to a bacterial issue, possibly Clostridium or E Coli in the GI, and that is what your vet is treating. In an ideal world, the immune system would guard against Giardia and keep it in check. If there is another infection that is stressing the immune system, Giardia may make an appearance. Another infection is probably what is going on and what your vet is correctly treating so the immune system can recover and quell the Giardia on it's own. But to directly answer your question on treating Giardia....my preferred treatment is Ponazuril. A second (less effective) treatment would be Panacur.
Randy