I see metal like anime, awesome stuff, but the fans just really ruin it for the rest of the folks who just want to watch and enjoy it damn it lol. I am not sure what @henchman24 means by "nerd" because I am not sure what the word nerd means anymore. It used to be the blokes who liked math and Star Wars, but now you get a push-up bra, wear cute cat-ears, and you can stream making thousands weekly. Are there genres of metal that are nerdy? Sure, I agree especially within the Power/Symphonic scene. Is it nerdy to shred neoclassical riffs and melt the Earth? Perhaps. However, I appreciate the musical talent that you find in metal and that is one genre you get more of the gifted musicians compared to any other genre not named Jazz. Good, Miley Cyrus can sing about partying in the USA and 50 Cent can talk about banging hookers all he wants, give me a stupid Alestorm song about getting drunk over those two any day of the week (not generalising all rap here). Or a Blind Guardian song talking about warcraft or whatever they sing about nowadays.
I might be going on a rant now, but the deeper and remote genres you reach, the more nonconformists are there to kick you out of the welcome mat. The darker and heavier stuff are the ones that the most unwelcoming and I think they make it that way, that's why Avenge Sevenfold created nicknames for everyone and that pissed off like 60% of their fanbase at the time.
I see "nerd" as a more general term, for people who are single-mindedly passionate about something they love. So you can be a nerd about anime or a nerd about death metal, or even, to be frank, a nerd about sports. It kind of runs counter to the jock/nerd dichotomy people think of, but really, is knowing the names of all the Avs prospects in other leagues really that much different from knowing the names of all the elves in Lord of the Rings? It has exactly as much effect on your life, after all.
I'm a nerd about a lot of things, heavy metal among them, but I would also consider myself a bit of a nerd about hockey as well. It's all part of how I approach my hobbies. Some of which fall under the traditional "nerd" umbrella, some that don't.
Metalheads, though, are a different breed when it comes to gatekeeping. I think there's a desire to keep metal underground and "special", a lot of metalheads take pride in being counterculture and different from others. I remember when Sabaton, for example, was an underground darling, but then they got popular, and now everybody hates them because they've got filthy casuals listening to them. There's no sin in metal greater than breaking into mainstream appeal.
It depends on the community you're in, though. For some reason r/powermetal is a really laid back community overall, with very few elitists that I've seen. But the old message boards I used to post on for heavy metal were just full of snobs and shitheads. I learned a lot from hanging around them, but a lot of them just weren't great people. I'm glad I've found some more wholesome communities that fit my tastes and attitudes a lot better than the places I was hanging out when I was a teenager. It's way more relaxing to be around people who are more into the music than the culture, because the culture can be toxic sometimes.