Tested with Firefox/Greasemonkey and Chrome/Tampermonkey.
Version for Safari:
I develop a similar script for the Rotten Tomatoes score, see cuzi/Show_Rottentomatoes_meter
and for the Letterboxd.com rating, see cvzi/Show_Letterboxd_rating
If no matching entry on Metacritic.com is found, you can type meta and a search field will appear in the lower left corner.