The standard of most commonly used instruments in the history of creating metal music usually consist of three basic instruments: the guitar, bass, and drums. I will leave out vocals for now and save it for a different topic on a another day.
I believed using those three is enough to create sounding material. Every time I'm introduced a new metal band that has other instruments embedded into their songs, for instance, a electronic keyboard or some foreign flute, my interest usually fades away after awhile. I just love the pure sounds of those combinations maybe that is because I'm old school.