Based on all of the available evidence, on any given building, a metal roof is no more or less likely to be struck by lightning than any other type of roofing material. In fact, a properly grounded metal roof, in accordance with the requirements of NFPA 780, will actually conduct the energy from a lightning strike over the broader area of the roof reducing the amount of heat transferred to the building and reducing the potential of damage do to fire. As an electrical conductor and a noncombustible material, the risks associated with use of a metal roof may even be a more desirable construction option. 

It should still be stated that to minimize the risk of damage due to a lightning strike, it may in some cases, be prudent to consider additional lightning protection. A lightning protection system provides for a continuous conductor from sky to earth (and vice versa) so that the electrical charge is furnished a path of least resistance through which electrical energy can flow, thereby reducing the risk to (electrically) resistive construction materials and human life.  

For additional information on the use of lightening protection, see NFPA 780 Standard for the Installation of Lightning Protection Systems. 2017 edition. National Fire Protection Association, Quincy, MA.