I think you're supposed to read these stories about halfway through your reading of the main series, but reading them afterward (as I did) worked fine. They fill in some of the blanks of the main series. The writing is mostly solid and occasionally inspired. The art, too, is solid and occasionally inspired. The Seven Evil Batmen from alternate universes are depressing and awful here as in the main series. Moreso, really, as we see the depth of their falls from grace. Recommended.
Dark Nights: Metal: The Resistance (2017): written by Scott Snyder, Benjamin Percy, James Tynion IV, Joshua Williamson, Rob Williams, Robert Venditti, and Tim Seeley; illustrated by Doug Mahnke, Howard Porter, Yanick Paquette, Jorge Jiminez, Jaime Mendoza, Liam Sharp, and others: There's no point reading this as a separate entity from the DC Comics Event series it supports, Dark Nights: Metal. You are supposed to read the stories collected here about four issues (of a total of six) into Dark Nights: Metal. Certain major things in that series are explored and explained. To say more would spoil that.
But yeah, you're going to need to read Dark Nights: Metal. And Dark Nights: The Road to Metal if you want to understand why Dick Grayson/the original Robin/Nightwing keeps having transdimensional visions.
Well, no, not exactly. Perhaps 2/3 of the volume comprises The Resistance. That follows the efforts of the Bat-Family, the Suicide Squad, and Green Arrow to figure out what the Hell is going on in Gotham City in the absence of the (real) Batman. The Seven Evil Batman are there. So too is Challengers Mountain, dropped into the middle of Gotham like a lawn dart made of sweet, sweet granite.
So, too, concentric circles patrolled by the minions of the Dark Batmen. Harley Quinn and Killer Croc get a lot of space to be their good-bad selves, fighting for Gotham even though they're mostly villains. It's probably about as fun as it can be, though it refers to events preceding the story-line that aren't collected in any of the four Metal volumes. Oh, well. Recommended.