Are you posting these names seriously? Or just taking the mickey?
If you're serious, then I can definitively say Metalla is not a nice name, nor is Metalyn. Why not get the name of a member of a Heavy Metal band and use it's feminine form?
For example (and these are just those bands I'm fleetingly familiar with - my brother was a fan you see)
The guys from Megadeth -
Dave,
Chris,
Shawn &
James - feminize them -
Davina,
Christi,
Shawna,
Jamie.
Sepultura -
Paulo - you could make that
Paula, or
Andreas -
Andrea.
Pantera -
Vinnie,
Terry and
Darrell -
Vinnie is fine for a girl, as is
Terri, and if its your thing
Darrell is OK too.
So don't make up a name, chose your favourite band and pick a name from there.