I think canonical collection of metal is a pack of albums, which would be just enough for listener to fully comprehend, appreciate and enjoy the Metal. Perhaps, even no other metal albums are needed to be heard aside from this collection. Collection of masterpieces and originators simultaneously:
Heavy Metal:Black Sabbath - First 6 LPs
Judas Priest - Sad Wings of Destiny, Stained Glass
Iron Maiden - First 5 LPs
Motörhead - On Parole, Motörhead, Bomber, Overkill, Ace of Spades
Angel Witch - Angel Witch
Hardcore Punk:Discharge - Hear Nothing See Nothing Say Nothing
The Exploited - Let's Start a War...
Speed Metal:Slayer - till Seasons in the Abyss
Sepultura - Morbid Visions/Bestial Devastation, Schizophrenia, Beneath the Remains
Metallica - Ride the Lightning, Master of Puppets
Thrash:Cryptic Slaughter - Convicted
D.R.I. - Dirty Rotten LP, Dealing with It!
Death Metal:Death Strike - Fuckin' Death
Possessed - Seven Churches
Necrovore - Divus De Mortuus
Autopsy - Severed Survival, Mental Funeral
Obituary - Slowly We Rot, Cause of Death
Deicide - Deicide, Legion
Morbid Angel - Altars of Madness, Blessed are the Sick
Atheist - Piece of Time, Unquestionable Presence
Infester - To the Depths... In Degradation
At The Gates - Gardens of Grief, The Red in the Sky is Ours
Dismember - Like an Ever Flowing Sream
Entombed - Left Hand Path
Grave - Into the Grave, You'll Never See, And Here I die...
Necrophobic - The Nocturnal Silence
Therion - Of Darkness, Beyond Sanctorum, Symphony Masses Ho Drakon Ho Megas
Unleashed - Where no Life Dwells, Shadows in the Deep, Across the Open Sea
Cadaver - Hallucinating Anxiety, In Pains...
Asphyx - The Rack, Crush the Cenotaph, Last One on Earth, Asphyx
Pestilence - all
Atrocity - Hallucinations, Todessensucht
Massacra - Final Holocaust, Enjoy the Violence
Amorphis - The Karelian Isthmus
Demigod - Slumber Of Sullen Eyes
Demilich - Nespithe
Morpheus Descends - Ritual Of Infinity
Suffocation - Effigy of the Forgotten, Human Waste, Breeding the Spawn, Pierced from Within
Incantation - Onward to Golgotha
Immolation - Dawn of Possession, Here in After, Failures for Gods, Close to a world Below, Unholy Cult
Gorguts - Obscura
Grindcore:Terrorizer - World Downfall
Napalm Death - Scum, From Enslavement to Obliteration
Godflesh - Streetcleaner
Carbonized - For the Security
Black Metal:Hellhammer - Apocalyptic Raids
Bathory - Bathory, The Return..., Under the Sign of the Black Mark Blood Fire Death, Hammerheart, Twilight of the Gods
Mayhem - De Mysteriis Dom Sathanas
Darkthrone - Under the Funeral Moon, Transylvanian Hunger
Burzum - Burzum, Aske, Det Som Engang Var, Hvis Lyset Tar Oss, Daudi Baldrs, Hlidjskalf
Immortal - Diabolical Fullmoon Mysticism, Pure Holocaust, Battles in the North, Blizzard Beasts
Emperor - In The Nightside Eclipse
Enslaved - Hordane's Land, Vikingligr Veldi
Gorgoroth - Pentagram, Antichrist
Ildjarn - Der Frysende Nordariket, Forest Poetry, Ildjarn-Niddhogg 2003
Sort Vokter - Folkloric Necro Metal
Dimmu Borgir - Stormblast
Beherit - Drawing Down the Moon
Graveland - In the Glare of Burning Churches, Carpathian Wolves, The Celtic Winter, Thousand Swords, Following the Voice of Blood
Summoning - Lugburz, Minas Morgul, Dol Guldur, Oath Bound
Varathron - His Majesty at the Swamp
Mütiilation - Remains of a Ruined, Dead, Cursed Soul
Antaeus - Cut Your Flesh and Worship Satan
Sacramentum - Finis Malorum, Far Away from the Sun
Havohej - Dethrone the Son of God
Averse Sefira - Battle's Clarion, Homecoming's March, Tetrasymmetrical Astygmata
Demoncy - Joined in Darkness
P.S. For complete canonical collection of Music, classical and prog-rock, and some electronic should be added to this list, but

let it be for metal only, as asked by topicstarter.
I think, any non-Metal people out there could just take this list of albums and anticipate for a great joy of discovery..