def api_key(endpoint = '')
      if private_endpoints.include? endpoint
        private_api_key
      elsif public_endpoints.include? endpoint
        public_api_key
      else
        fail "No such endpoint #{endpoint} available."
      end
    end
def api_key(endpoint = '')
      if private_endpoints.include? endpoint
        private_api_key
      elsif public_endpoints.include? endpoint
        public_api_key
      else
        fail "No such endpoint #{endpoint} available."
      end
    end
Decide which api key should be used: private, public