I use a cheap (A$2) bonding agent with names such as 'Bond-Crete', or this one in the photo is 'KS Bond'. You can find this type of glue at any "cheap" or discount store (i.e. the $2 Shop, The Reject Shop, etc.). I use this glue to adhere metal to almost everything.
I make my own jewellery and I have not had one glued item come unstuck yet. And I've been using this type of glue for as long as I remember. I have used this glue on items I fit in my 'Fairy Garden' as well, items which have sat outside for over 2 years. The glue is still holding, even after rain, freezing temperatures and moss growing over everything.
The description reads:
Strong adhesive. Chloroprene cement.
Use: Bonding vulcanised rubber and leather. Metal, stone, glass, porcelain, sponge, paper, textiles, wood and plastic to themselves or each other.
