In this Wikipedia link, the definition of exciton is given by
An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and in some liquids. The exciton is regarded as an elementary excitation of condensed matter that can transport energy without transporting net electric charge.
Why such a bound state (which I believe is a Bosonic state) cannot exist in metals? Coulomb attraction between electron-hole pairs should be present in metals too. The link I cited, doesn't explain this point.