static_accessor.rb

module MyLib
  class MyClass
    class << self
      attr_accessor :api_key
    end

    def self.my_method
      puts "key: #{api_key}"
    end
  end
end

module MyService
  class ModuleA
    def self.get_my_method
      MyLib::MyClass.my_method
    end
  end

  class ModuleGeneric
    def initialize
      MyLib::MyClass.api_key = "super-secret"
      @module = ModuleA
    end

    def get_my_method
      @module.get_my_method
    end
  end
end

s = MyService::ModuleGeneric.new
s.get_my_method