/ thread[BMoonsorrow (any album; these guys have tons of long songs)
Vital Remains has some longer ones.
I also think, even though you're specific about metal here, you should also keep in mind some classic prog rock and fusion out there. When I need to take that kind of journey (which is often), I usually go there. It's a shame there aren't more long pieces within metal, but you can still find very musically, instrumentally challenging and atmospheric stuff within progressive and experimental rock-related music.
But often without the preferable instrumental breaks the op was looking for. Some songs that do have em...Forest of October has the longest one I think, then In Mist She Was Standing and Deliverance end with like 3 minutes of instrumental, Morningrise is good for that too of course.