I use a cheap ($2-aus) bonding agent with names such as 'Bond-Crete' or this one in the photos is 'KS Bond'. You can find this type of glue at any "Cheap" or discount store. (Ie: 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 fit my 'Fairy Garden' which have sat outside for over 2 years. The glue is still holding, even after rain, freezing temperatures and moss growing over everything. 
Description reads: Strong adhesive. 
Chloroprene cement. Use: Bonding vulcanised rubber and leather.  Metal, stone, glass, porcelain,  sponge, paper, textiles, wood and plastic to themselvesor eachother.  (https://i.sstatic.net/TSHpz.jpg)