Metallic Dragons are one of the many specific breeds of dragon native to Dungeons & Dragons and Pathfinder. They are one of the two most iconic family groupings of D&D dragons, alongside the Chromatic Dragons. Lesser D&D dragon families have include the Catastrophic Dragons, Gem Dragons, Oriental Dragons (or "Imperial Dragons", for Pathfinder), Planar Dragons, and Epic Dragons.
In an expansion of the alignment mechanic, chromatics helped define what our frenemies on TVTropes calls "Color Coded For Your Convenience", where the general color-scheme of a dragon immediately helps you figure out what they are as a general rule.
Metallic Dragons are based on the colors of various metals and are traditionally portrayed as the "goodly dragons". There has been some contention about this over the years; as most metallics are "noble" metals like copper, gold and silver, whereas "baser" metal-based dragons were given a tentative life as the
Ferrus Ferrous Dragon sub-family in Dragon Magazine #170 and #356, for AD&D and 3rd edition respectively. In 4th edition, meanwhile, WoTC pointed out that "Always Good" creatures are kind of less than useful to DMs, since most parties won't play evil campaigns and there's only so many times you can use "the super-intelligent and ultra-wise being of good is misguided/a dick/incorrect/mistaken" before it feels hollow. So they changed the default Metallic alignment to Unaligned and portrayed them as usually meaning well, but being arrogant and inhuman and so having lots of believable reasons to get in the way of heroes without being the malicious monsters of their chromatic kin. Using them as questgivers or mentors doesn't seem to have occurred to anyone.
The patron god of all Metallic Dragons is Bahamut, whose status as being based on platinum makes him unique.
They can crossbreed as revealed in the 3.5e draconomicon but not as often as chromatics. The dragons themselves usually have two breath weapons i.e silver having ice breath and paralyze gas.
Now i know your wondering does this mean dragons can crossbreed with other dragon types(i.e chromatic+metallic). The answer is yes, but the child is usually viewed as an abberation and hunted down. That said, it is possible for a dragon to care for it, but thats extremely rare as each dragon is unique after all.
The original five
The iconic quintet of Metallic Dragons in D&D goes: Brass, Bronze, Copper, Silver and Gold, ascending in power (at least from copper through silver to gold). Unfortunately, this quintet hasn't entirely sunk into the fantasy consciousness the same way that the Red/Blue/Black/White/Green quintet has. At the very least, it rather throws off the theme when three are noble metals and two are copper alloys, though at least all five metals have historically been used as coins so the theme does make sense with some thought.
Chatterboxes. Brass Dragons prefer hot dry areas like deserts. Brass Dragons like nothing more than sunbathing in a good spot and prattling with a stranger for hours on end, even if they have to bury them up to the neck in sand to do so. They favor things that can talk in their hoards like sentient objects and Genies bottles. Brass Dragons have big head crests, and kind of look like dinosaurs. They are the everyman/socialite of metallic dragons, talking their way out of trouble and building networks of contacts and informants. If you want to include a dragon in your campaign, but aren't sure how throw in a Brass Dragon quest giver.
Honorable champions of order, but prone to Lawful Stupid. Also live on coasts, makes good fluff for Dragonborn marines.
The Copper Dragons are most known for being the practical jokers of the Metallic Dragons.
Regal and imperious, Gold Dragons traditionally hold the position of being the mightiest of the Metallic Dragons, although that has been shaken up before - most notably in 4th edition. Originally, Gold Dragons were designed to more of an "eastern dragon" body-structure, with long, serpentine, wingless bodies and flowing mustaches. Their bodies have changed to match the standard "western dragon" archetype, but the pseudo-mustaches seem here to stay.
While the copper, silver, and gold dragons all have the ability to polymorph themselves into humanoids and have sex with live among mortals, silver dragons are noted for being by far the most fond of doing this. If you come across a creature with the half-dragon template, it probably got its dragon half from a silver dragon. They're basically the furries of the dragon world. There's even an entire human subrace, the Silverbrow humans, who are the result of a LOT of human-on-silver-dragon action, which you can't say about any of the other types of dragons. What we're trying to say is that they're slut dragons.
Other metallic dragons
Whilst a few additions to the Metallic family tree have appeared here and there - mostly in the Forgotten Realms - it was 4th Edition that took the stance of shaking up some sacred cows. Pointing out that 1: Bronze and Brass threw off the Noble Metal theme of the "core" metallics, and 2: Bronze, Brass and Copper have always been hard for anyone except the most fanatically invested neckbeard to tell apart, they removed both Bronze and Brass, delegating them to a later appearance in the Metallic version of the Draconomicon and replacing them with new Adamantine and Iron Dragon species. similar dragons called ferrous dragons... which is weird alloys would be a better name covered somewhere else
Adamantine Dragons are heavily-armored Metallics native to the Underdark, who may arguably be more powerful than even Gold Dragons, with distinctive beak-like snouts and a breath weapon that deals thunder damage. They are essentially the Metallic analogue to the Purple Dragon, favoring the Underdark as their domicile of choice. These dragons were entirely unique to 4e; AD&D had featured an Adamantite Dragon, but that was a Planar Dragon native to the Twin Paradises of Bytopia.
Fickle, whimsical and highly chaotic, the Mercury Dragon is an obscure beastie that is native to the Forgotten Realms, having first appeared in the original Faerun-set Draconomicon before hitting the big leagues with a place in the AD&D Monstrous Manual, followed by 3rd edition's "Dragons of Faerun" and then the 4e Metallic Draconomicon. Traditional Mercury Dragons use laser beam breath weapons and reflection-based attacks. The 4e version is a superb shapeshifter that might as well be made of living metal - picture a draconic T-1000 Terminator - and wields a poisonous breath weapon.
The Spelljammer splatbook Practical Planetology introduced Mithril Dragons, which are endemic to the tidally locked planet of Radole. They are Chaotic Neutral, and unlike other dragons, have no desire for hoarding treasure, instead spending their lives trying to outdo each other's sick aerial maneuvers on the molten sunward side of Radole. Their breath weapon is a beam of blinding light.
4th edition's Mithral Dragons are the elite of the Metallic Dragon family tree, native to the Astral Sea and wielding a diverse array of powers that allow them to bend space and time to their will. In this, they could be argued as a spiritual inheritor to the original Adamantite Dragon, but their arrogant self-righteousness and desire to change the world - regardless of what the world thinks - is perhaps closer to the 4e rendition of Couatls.
Entirely unique to 4th edition, Orium Dragons are red-gold colored dragons, with serpentine heads, necks and tails mounted on a rather feline body frame. Based on "orium", WoTC's trademarkable name for "Orichalcum", they are obsessive historians who seek out ancient ruins to maintain, preserve and restore. Their unique breath weapon is a gout of toxic vapor, which then coalesces into a snake-like construct that keeps on fighting on its own after being exhaled.