If it was intended, then essentially it's a "jump through hoops to achieve expected power levels" thing. If a higher level warlock is expected to carry around a bag of rats, or a surrogate, and cast, kill, rest so that he's at the baseline intended power level, that's just bad design. Give the warlock hex for free at the beginning of the day and just move on. No need to waste play time to achieve maximum potential.
It's an exploit that requires specific thinking (bag of rats is a clue) to achieve. If the rule wasn't in place, there's no way any warlock player would consider waking up, casting hex on a rat, killing the rat, and then taking a short rest. It's only the exploit that makes that attractive, and only if your interest is to game a minor benefit. Hence the thinking of the game as a game.
I should have specified a couple of things, but as "i can't think" meant " i have no time to", so please excuse if i was sort of brief and ended up cutting short important parts of the question.
I meant to ask how is the ability to mantain Concentration over a Short/Long Rest - that is the major issue - more than the particular situation of bag of rats + Warlock + Hex - that is an exageration - is bad design or an exploit and metagame.
ANYWAY.
How does the fact that the rules model the world of D&D to allow such a thing "metagaming"? It is an exploit - by definition you are using the rules at your advantage. But so is the character doing. It's not metagaming more then a fighter attacking over pulling a lever because he knows he is better at attacking. It does not make that anything worth chastizing.
Using wings to fly is nothing more than playing the game - bringing plans to open an industry producing airplanes is bad metagaming, however - You make your character act with knowledge he should not have.
The player is exploiting rules, but from the character perspective it's something that is just a natural conclusion of said rules. The rules work this way (if the DM allows).
If i found out that not eating meat makes me feel better, why should i eat meat? If i find out that charging the phone to 99% makes my charge last longer than 100%, why should i charge it to 100%? If i found out that not eating meat while charging the phone makes me go 1 mph faster on average, why should i not do so? Prehaps i would go over the speed limit, and then it would not be a good idea... but if i can, why not?
Naturally i'm not using borrowed powers from near godlike entities but if the electric company or the Automobile industry were to get this secret, there would be repercussions. Prehaps. Still it's not "gaming the rules" anymore than what i know that i can get away from. And again, people try every day to break the limits of our percieved existance. Flying machines are common nowadays because someone took on themselves to challenge the limits of the rules themselves.
If the DM says "ok", there's no metagame with your meaning: The character already knew how the things worked. If the DM says "no", the thing simply won't work: the rules do not work that way, the character might have tried because things apparently could but don't, or didn't try at all because the rules were clear and known and tested.
Or better: Every single action a player makes the character take IS METAGAMING WITH YOUR MEANING.
Metagaming is necessary. It's part of the game. It's intrinsic in this sort of games. The character, in this case, already has said knowledge. How is this metagaming using this knowledge? How is me playing my character "metagaming" because he has the ability to know and piece together such informations?
I'm an Evoker. Magic Missile is evocation, i have an ability to do make each missile do more damage, i'm using magic missile to destroy a single target. Or i'm unable to because that's a calculation that my player did and end up being better than other options?
I'm a Fighter. I can grapple people so they can't move. I can also put people down in prone position so they have to get up to be effective. I can do both! Or not, because that's a calculation that my player did and end up being better than other options?
I'm a Warlock. I can Hex a creature and, as long as i maintain concentration, the spell continues lingering even after the creature is dead or very badly beaten, at which point i can redirect the effect to another creature. I regain the ability to cast my spells with just a little rest. I cannot do so to any kind of creature between when i wake up and breakfast because that's apparently something that it's part of a cosmic game and not something that i live day to day.
It's not a reason, really, expecially compared with other possible reasons - you can't regain your spells you are concentrating on | you can't rest while concentrating | you can't do so because your patron would not allow you to waste his powers on such an insignificant task.
It's metagaming - no, it's not. It's just playing the game, same as every other possibility ever.
You regard it as an exploit - i agree. In the broad sense it's an exploit, same as the unsleeping Sorcerer. And I would not allow the bag of rats every morning same as the unsleeping sorcerer, or the Simulacrum chain or whatever - and mind you i still allow all of this, just consequences are harsh. If the player manages to make a good point for the situation and is ready to face consequences: Go ahead. You have been warned and told what the consequences could be and why. It's a limited use card in your book, use it wisely and all will be ok.
But metagaming? I really can't see how this is metagaming any more than everyting else that normally happens in a D&D game, starting from "i'll attack now since i'm level 5 and i have extra attack so i attack twice". Heck, planning progression for multiclassing is usally a worse offender!!!
So... why pointing it out? It's just like saying "because it's bad design and part of how the game plays out"
Prehaps it's just me thinking that pretty much the "bad" metagaming is making a character act on informations he could not have (and just the first example i can think of right now) and can't see how this kind of metagame is not to be expected to any extent.