Tested with Firefox/Greasemonkey and Chrome/Tampermonkey.

I develop a similar script for the Rotten Tomatoes score, see cuzi/Show_Rottentomatoes_meter

Contributors:

Supported sites and products:

• Bandcamp
• Apple Itunes (only Music)
• Amazon (only Music, Movies and TV Shows)
• IMDb
• Google Play (only Music and Movies)
• Steam
• GOG
• Epic Games Store
• Gamespot (only PS4, XONE and PC games)
• Rotten Tomatoes
• Serienjunkies
• BoxOfficeMojo
• allmovie.com
• fandango.com
• Wikipedia (en)
• themoviedb.org
• letterboxd
• TVmaze
• TVGuide
• followshows.com
• TheTVDB
• ConsequenceOfSound
• Pitchfork
• Last.fm
• TVNfo
• rateyourmusic.com
• nme.com - New Musical Express
• albumoftheyear.org
• itunes.apple.com
• epguides.com
• save.tv
• wikiwand.com (only Movies and TV Shows)
• Radarr - you need to set your server address in the script header, default is @match http://localhost:7878/*
• trakt.tv

Screenshot:

If no matching entry on Metacritic.com is found, you can type meta and a search field will appear in the lower left corner.

https://github.com/cvzi/Metacritic-userscript