create_metafield_map.rb

def Shopify.set_product_metafield_map(id, namespace, key, map)
  # convert the map to JSON so it fits in one metafield
  data = {
    "metafield" => {
      "namespace" => namespace,
      "key" => key,
      "value" => map.to_json,
      "value_type" => "string"
    }
  }
  request(:post, "products/#{id}/metafields.json", body: data.to_json)
end
  
> Shopify.set_product_metafield_map(1234567890, "product", "custom_attrs", { "width" => 11.2, "height" => 35.7 })
=> {"metafield"=>
  {"id"=>10030144453,
   "namespace"=>"product",
   "key"=>"custom_attrs",
   "value"=>"{\"width\":11.2,\"height\":35.7}",
   "value_type"=>"string",
   "description"=>nil,
   "owner_id"=>1234567890,
   "created_at"=>"2015-11-20T16:48:03-05:00",
   "updated_at"=>"2015-11-20T16:48:03-05:00",
   "owner_resource"=>"product"}}