I'll 'second' the Stalker series - esp. Call of Pripyat. All of them are 'open world' (although not one single 'continuous zone' - there are 'zones' - but they are large and you can travel back and forth between them), and you have a lot of freedom in how and when you do things (and to go do your own thing and ignore the 'main quest' for a bit if desired).
There are also a ton of 'full campaign' mods - that will give tons of hours playing more and new stories in the 'exclusion zone' (that's where the story takes place).
Very open and fun RPG'ish FPS. One of my favorites.
As far as the Metro series - I only know the one here on GOG;
Metro Last Light. I picked it up on on 'day one' here on GOG (based on how much I love Stalker) - and to be honest - have more or less lost interest in it - it seems to be a 'corridor' game, very linear, with plenty of QTE's, and not 'open world' at all from what I can tell.
By 'corridor', I don't literally mean 'it's all in corridors'. There's outdoor areas and so forth - but each area seems to be a self-contained 'mini-theme-park'. You cannot go back and can only go forward the way the story propels you. For example - you get guided to an outdoor location - and while there can only just do the task told to you (like 'kill the three thingys'). No real exploration - no going back and coming back later. No going forward until that 'task' is done - then you are 'propelled' along to the next mini-them-park. Each area also seems to have a 'you went thru the magic unmarked 'next-zone' door - and now it's locked behind you - you can't go back - even if you weren't done looking around (and tragically - that same door 'model' is also used 'in-zone' so you can't really tell when you are facing one). Metro has been very linear so far - way more than, say, HalfLife - where you could at least tell you were going to 'zone' and could wait an complete your area checkout first. Unfortunately I've found it disappointing, so far.
Definitely look at Stalker :)