Not really, just as I don’t have to be looking up the stuff I deal with every day (or year as a retired teacher) you and I aren’t either out adventuring every day or out playing D&D all day every day so the monster stats aren’t top priority for us (even if we do get a decent general concept from years of playing). However, for adventuring hero’s in any specific world knowing a great deal about their own world and the monsters and foes they might have to face would be stuff they would be trying to learn and/or already know. The should know the major factions and political groups of their region, something about the various intelligent species they are likely to deal with etc. Taking the  FR sword coast as an example (because I do know a little about it) they should have decent knowledge about groups like the zhentarim, iron throne, lord’s alliance, harpers  and political groups in and around the major cities. They should know that most trolls are damaged/killed by fire or acid, the physical differences between orcs, goblins, hobgoblins and bugbears as well as elves, dwarves, gnomes and halfling. They won’t be able to tell a Sarrukh from a Yaun ti or know what a Phaerimm is - except dangerous - at least in their 1&2 but as they get more and more experienced they should be learning more about the different monsters both from their own experiences and from downtime activities ( including “carousing”) ( bard’s tales etc - may be exaggerated but still have a core of truth.) overall they should be as expert in their world as we are in ours. So while they probably shouldn’t be asking to see the monster manual etc in the middle of the adventure anything they can remember from outside reading should be legal ( just possibly incorrect for this specific case).

The DM in the one PbP campaign I’m in has a simple system. If, in their opinion, the monster is something we would know about in that level of detail they insert the tooltip for the monster. If, in their opinion, the heroes wouldn’t know about those monsters to that level of detail then they don’t. If I get a tooltip I look up the monster. If I don’t get a tooltip I do my damndest to play as if my PC has absolutely no clue what she’s fighting.

When it comes to metagaming, there's always the good, the bad, and the ugly. The good mostly find ways to mostly avoid or explain, the bad use meta gaming at least to the advantage of the party and get it when told to cut it (at least for a while), and the ugly try to "hack" your game ftw and get a meltdown when you drop the hammer on them.

No and you know it, what they have done is paid attention to stories and have heard of phase spiders and know hey are badder versions of giant spiders that can phase in and out of the prime material at will. Think seriously for a few minutes about how much “stuff” you know in various areas that others don’t - because you are working in that/those areas or happen to have ( or have had in the past) a serious interest in them that you know but most others don’t - adventurers are going to have the same sorts of extra information about their world and the things they need to know as adventurers and much of that is in the monster manual so if they can Romberg it and use it it’s not really metagaming to my mind.

In our worlds, we and our DM assume we know a bit about the more populace monsters and such we find in our adventures.  Each character also has certain types of monsters, or in some cases, region specific information they are likely to know.  We are often allowed a free History check if it seems likely we may have heard, or read something about an enemy we're facing.  In many cases, we, the Player, know what the monster is about and on a moderate roll, the DM will let our character remember some detail about the creature, but not all, unless we roll really well.  Thus we end up with a piece of extra info that will help us in the fight, but many times other details, maybe a resistance or vulnerability not mentioned.  We, as a group, even if some players know better, play as though we don't and have had one of our casters use a fire spell that I, as a Player, knew would be futile.

I feel blatant metagaming does take a lot away from the fun.  The first time we encountered Shadows, some of us knew they had a special attack.  NONE of us started the fight as though we knew.  No fear, no hesitation, no extra attempts to make sure we weren't hit.  It's more fun to PLAY the character, who is clueless to the dangers sometimes, than to always be omnipotent.  

Quote from Wi1dBi11 >>

No and you know it, what they have done is paid attention to stories and have heard of phase spiders and know hey are badder versions of giant spiders that can phase in and out of the prime material at will. 

 There's a huge gulf between "phase spiders can phase" and looking up a stat block. You are correct that seasoned adventurers would probably know the information you stated. But that's not the issue here, because the commenter that started all this watched it phase. That's how they knew to look up phase spider. So they're not going for the broad strokes that you're describing (because they already got those), they are trying to zero in on exact details like AC and hitpoints. That is the kind of harmful metagaming that people are talking about.

Yes that kind of metagaming is not acceptable - but that isn’t metagaming it’s straight up cheating - they are not role playing as those stat blocks are not in world details they are game mechanic values. Yes we sometimes shorthand combat discussions and speak (semi orc) about the AC and HP of the PCs and monsters but no one but the DM should actually look them up in game. I might tell a player who asked that it had a chitinous exoskeleton that their weapon should cleave easily on a solid hit  ( where say an alligator has a thick leather and bony skin that they are going to have problems cutting or stabbing thru. You shut that (stopping the game to look up stats) by giving them the 6 seconds of the round to say what they are doing and then they are stuck with it ( if it’s a spell they have to say which one then if we have to look it up for details it’s ok as they can’t change it once they’ve said it so they get what they get.

I think knowing something, mainly threats, exists in the world I or a PC lives is one things.  Presuming a Janes Defense Encylopedia level or even an Osprey's book knowledge of such threats is a steep ask on the part of a player for a PC.  Everyone knows "fire bad", but not everyone knows which types of extinguishers you're supposed to use with which types of fires (or that those markings making the distinctions on the extinguisher even exist), and even if they know the type, they may not know to work the fire from the edge and instead "shoot" the extinguisher at the fire's "center mass."

To take the phase spider example, how "learned" a PC may be about a given monster comes down to DM discretion and interpretation of the world building.  Sure if there are communities that neighbor places where phase spider's nest to the point that the communities have regular phaser hunts or what have you, they may know a thing or two.  On the other hand if a far and wide traveller's tale is the so source of a PC's knowledge of such a monster, recognition isn't necessarily any level of practical knowledge.  For all the PC knows the far and wide traveller took it out with two crits w/ SA damage, which would lead the PC to underestimate the threat.  

Rangers, mechanically/design speaking are sort of given a privilege against these assumptions, depending how you play them.