WARNING: benji post; with some extended wrap-up thoughts re: METALSince I'm hopelessly behind on all things DC, I'm just going to read the entire Metal mini-series and its tie-ins rather than read up until the event starts. Doesn't really look like there's too many tie-in books anyway.
The series/tie-ins reference events up to the start, especially Snyder's Batman run (which ended with New 52) and the Court of Owls. But it's not essential to the METAL story itself other than this, the Court's actions in an earlier plotline were not just part of torturing Bruce, but to set him up. Also, there's some tie-ins to DARKSEID WAR, when Bruce had Metron's chair he learned a bunch of stuff about the universe, that's why he has The Joker in a cell under the Batcave. (Also, this story is why Darkseid is a baby.) The tie-in issues also have references to prior events, almost all of these get enough of an explanation that you don't need to know the original, there's lots of "random thing before now means something" type stuff.
All the rest are gravy references to place METAL within the DC Epic.
The main one that METAL doesn't explain, isn't Snyder related and not in anything from New 52 is why Nightwing's head is slashed open and he's having visions. It's Dr. Hurt/Thomas Wayne (from RIP) who showed up for two issues to slice Nightwing in the head with a specific metal and sound like he was going even more mad, when he was actually foreshadowing Barbatos' plan which started like the next week in METAL. Snyder obviously wasn't aware of this or didn't give a shit because even in METAL the plotline doesn't go beyond the tie-ins written by the Nightwing writer and Nightwing basically doesn't do anything or help.

I liked Metal a lot, but man was that last issue rushed. It at least needed some extra pages, if not a whole extra issue.
It was already an extended issue, 48 pages, and it's received at least three months of delays. METAL was supposed to be basically over by the time 2018 started, and Doomsday Clock would pick up the banner as the EVENT, except the delays to that are why we're getting NO JUSTICE.
Problem was, fell back to Snyder writing a big superhero fight for 20 pages, and then a tighter and better done Batman/Joker showdown of sorts. Followed with an epilogue to actually explain what happened.

Also DC writ large decided to use METAL to do some tinkering with existing series, as well as shuffle in and out creative teams. Which is a bit of a response to:
With DC, I've always operated under the assumption that anything Justice League-related isn't actually happening because it almost always seems to take place within it's own little world. JL books or events never really bleed over or tie into the other books or series in a meaningful way. Metal and the whole No Justice thing seem to be continuing that tradition. I'm still eager to see where Justice League goes from here though. I just treat it like an Elseworlds thing.
DC books have been for a couple months now been taking place "post-METAL" as there's been plenty of references to it in the past-tense. The delays were apparently not due to Snyder, so the storyline was known to the other writers but they didn't want to tip anything about it. Like Batman's proposal to Catwoman is the result of his coming back from METAL I believe. But King until recently couldn't outright say that was the reason.
As noted, DC is coming out of METAL with far more changes than were originally planned (just these new series like The Terrifics, etc.) because the delays created a post-METAL gap that was never expected to be there. METAL would have ended and things gone back to normal as Doomsday Clock continued to unfold. But METAL got delayed, Doomsday Clock got delayed even more. And Bendis/creative teams.
This has come together to where they were going to relaunch the JL sets originally this summer, including bringing back Dark and having a Suicide Squad/JL-mashup team, but what DC seems to have done is asked the METAL team to create a new event to explain the new Justice League teams, which is NO JUSTICE. The four teams in the teaser and in the series are not entirely the JL teams that will come out of it, but are close. Like Wonder Woman will be leading JLDark, Harley and crew will be in the one JL team with a tech focus (iirc), Martian Manhunter is into the main JL book which will take on a space/global focus, Batman's JLA team will be the "ground" based team. With all of them taking on JLA's "redemption" concept which is why they have villains/anti-heroes in them.
Basically METAL got upgraded to a larger scale EVENT that DC has lined up a bunch of changes to "come in the wake of" that go beyond the six series they originally intended to launch. (Two of which are already two issues in lol, while another, I think DiDio's book is like a year behind now.)
By the end of Metal I didn’t really understand what was happening or why. The earth fell below the darkness or something because Batman got bukkakeed by 9 metals.
The only part I really found interesting were the tie-ins. The actual series sort of sucked. It was just too confusing, and not well explained.
Snyder wrote a Morrison Epic but he's not the level of writer to provide the density where re-reading makes it clearer, METAL basically doesn't get clearer on re-reads once we get to the events of METAL proper. It's by and large a loud mess and reveled in that batshit loudness until this last issue.
I'll make things worse by trying to explain some of it for you while also working through my thoughts on it and where it went.
The Dark Multiverse, exists
literally underneath our standard universe and is where corrupted stories go to die. Stories with endings that wipe out everything, bad dreams that aren't fully formed and so collapse, twisted versions of our heroes, etc.
Batman discovered that the supernatural metals of the DC Universe were pointing to a greater purpose, which was that they can open the gateway to the Dark Multiverse, especially because the original "thing" at the helm of The Forge has disappeared. (Which allowed Barbatos to trap Carter Hall as his Forge.) The entire first half plot was based around how only Batman could put this together and get it to the right point (partly because of when he traveled through time post Final Crisis), where the agents of Barbatos could trick him and open the gateway.
After that Earth (which is the center of the Multiverse) began to
literally sink into the Dark Multiverse. (Which again, is below the Multiverse. Literally below it.) If it did Barbatos would end "good" stories. End heroes. End hope. The Trinity overcame this by finding the Tenth Metal (which arguably should be the eleventh since BATMANIUM! HOW DID THEY FORGET BATMANIUM!) within The Forge which was the one thing that could stop the Dark Multiverse as it's literally "God's metal" of creation.
spoiler (click to show/hide)
The Batman Who Laughs part in the finale was the backup plan for Barbatos, which if he couldn't have the universe, no one could. They were going to merge The Monitor and the Anti-Monitor (well, his brain) to destroy all reality.
The "antenna" stuff was a call back to the first two Crisis in which the big bad's built antennas that the heroes then used to turn the tables. The Thule was a throwback to Final Crisis (and The Multiversity), Barbatos to The Return of Bruce Wayne.
The Epilogue infodump is actually the end game of METAL because the two pages where the heroes were all holding hands and shit and they cracked the Source Wall and God's hand appeared, Barbatos (or something) used the connection powered by literal metal of creation to make some of his terrors (and maybe even worse in the case of Atlantis, some people's hopes) real before he was bound.
And for the upcoming NO JUSTICE, the breaching of the Source Wall called the attention of billion year old celestials. (I use celestials here because not only do they fucking look like Marvel's but "METAL", they sound like the same exact concept, each one an embodiment of a concept.)
This ultimately was the part I liked and disliked most about the finale. (Other than Snyder closing his take on The Joker by making him essential to save the universe.) I disliked that Snyder shoved that all into an Epilogue. And infodumped it over two pages. I liked that Snyder realized he couldn't come up with a more powerful metal AND pull off a stunt to revert the universe without it having consequences of all types.
From a canon wank perspective, I disliked that he's tied it to breaching the Source Wall without doing any of the canon research he did for everything else. The Wall has been breached multiple times before, including by Batman of all people, and it's been canon even before Wildstorm was integrated that The Bleed is on the other side of it. Morrison changed this in The Multiversity for a specific reason. (He also renumbered the Earth's, so there's no Earth 52. The Earth 53 Snyder had come to the rescue skips an Earth!) The Multiversity explains what's on the outside of the new Source Wall. Other multiverses. As The Empty Hand has come from recently destroying Multiverse-2. Snyder appears to be using it to instead bring back things from the DC Canon.
And it's through exploring this that I come to METAL's central problem. Barbatos. Once the Dark Batmen are shoved aside, outside of the Batman Who Laughs, so that the heroes can confront a literal Barbatos is when the epic falls apart. When they're battling Barbatos as a concept, the story takes on its epic apocalyptic tones and the heroes are seemingly doing more than punching even as everything fails until they realize what they're battling. And this is why I called it an attempt an a Morrison Epic. Morrison constantly makes his ultimate villains a concept entity. Darkseid goes beyond a super tyrant, an alien power, even an Evil God, instead he's the personification of No Free Will. His physical "defeat" by Batman in Final Crisis, is what brings about the All Over, as he's dragging down reality with himself. The Empty Hand, exists, along with his Gentry minions, as concepts taken to an unending horror. Barbatos should have been and stayed this, the embodiment of nightmares, a creature literally created by our nightmares and fears, who has become so empowered because of Batman's rejection of this. The Dark Batmen exist because Batman didn't succumb even in those nightmares including the Snyderverse one and including the ones where he had to trap his friends in hell to survive.
By the end we're seeing Dark Wonder Women, etc. and some are drawn interesting on their own, but unlike the Dark Batmen, we didn't get their backstory which was way more effective than I thought. The story never grapples with The Dark Batmen, outside of one, not being servants of Barbatos, but instead trying to save their universes. Arguably for five pages of wanking about heroes and metal and hope as white swirls around, it would have been more effective for the heroes to re-emerge in the Dark Batmen, to reject Barbatos seeing what his endgame actually is, shattering Barbatos' hold on the nightmares and requiring him to fall back on the one who wasn't "broken" but chose willingly to "break" in The Batman Who Laughs.
This is also a way you can get to the deus ex machina of the "Tenth" Metal and do it all on panel. The first half of the series and the one-shots touch on it, but the story mostly loses it, WHY THE FORGE EXISTS and why it's in the Dark Multiverse, etc. And you can use it for the same reason. The ended stories are being destroyed in The Forge to provide the fuel to create new stories. A sacrifice of the other Dark Batmen to provide the fuel for Batman, Superman and Wonder Woman to come back from within The Forge with the "Tenth" Metal that essentially ends Barbatos' plans and ties up the "metal" and Dark Batmen's plotlines. Something METAL doesn't actually do. Batman and The Joker still need to do the one thing that only they know, which is track down their counterpart, to end the backup plan, AND get The Monitor, who happens to know how to pull the Earth out of the Dark. But most importantly it shifts the deus ex machina to being one that actually deals with the concepts of The Dark Batmen and Barbatos, by defying him, they provide the fuel to return our heroes, it's an inverse of the start when our heroes are used to open the door to Barbatos and elevate the Dark. It also firmly plants METAL as a story about heroes/hope/dreams/aspiring vs. terrors/fear/nightmares/despair.
Overall, I have loved METAL though, even when it sprawled all over, it was in a fun way. But it never really escaped Snyder's heavy hand, which is why some of the tie-ins like Batman: Lost and Wild Hunt were more engaging than stuff in the main book. (I have to assume Snyder's buddy Tynion saved a lot of these, as did their buddy Williamson, as they're credited on the stronger stuff...thankfully they're both full credits on all of No Justice!)
One thing kinda weird to me is that Snyder didn't used to write so clunky. American Vampire (until Second Cycle) is tightly plotted and handles its scope better,
The Black Mirror start of his Batman career is brilliant and even into Court of Owls does a lot more with its plotting, Death of the Family doesn't get out of control and sticks to its central hook even, up until Zero Year which is where it falls apart. And Eternal is a mess. Both of them could have been half as long. Endgame is a mess that only seems tightly plotted because it's six issues, and even there its brilliant moment of OH SHIT WE SERIOUS is earlier and ultimately irrelevant. Even The Wake seems okay, and Swamp Thing was fine, Rotworld is...actually...Rotworld is sorta like a METAL test-run, now that I think about it...hmm...huh. Interesting.