def add_tag(tag)
      if @counters[:attributes][:tag] >= Mailgun::Chains::MAX_TAGS
        fail Mailgun::ParameterError, 'Too many tags added to message.', tag
      end
      set_multi_complex('o:tag', tag)
      @counters[:attributes][:tag] += 1
    end
def add_tag(tag)
      if @counters[:attributes][:tag] >= Mailgun::Chains::MAX_TAGS
        fail Mailgun::ParameterError, 'Too many tags added to message.', tag
      end
      set_multi_complex('o:tag', tag)
      @counters[:attributes][:tag] += 1
    end
Add tags to message. Limit of 3 per message.

 @param [String] tag A defined campaign ID to add to the message.
 @return [void]