AFAIK metal roofs are not usually grounded and do not attract lightning strikes any more than asphalt shingles; nor do metal roofs protect a building against lightning -- but that's just opinion... I'm not an expert. AFAIK, the only way to protect a building from lightening is with a properly designed and installed lightning protection system.
As a side note, a long time ago in a bayou far far away (actually it was the UF Lightening center), I heard an interesting theory about grounding masts on sailboats backed by a dataset too small to draw accurate conclusions from. The theory went that a grounded mast had less resistance to water, so was more likely to be struck by lightening. But, if lightening hit an ungrounded mast, it would do massively more damage to the boat - so it was still worth while to have grounding.