How to add a tag when messages is multiline in Logstash

Question

I use Filebeat6x to ship my logs to logstash. Some of my logs may be a multiline thats why I use Filebeat to Manage multiline messages

Now I want to add filter in logstash to do something like

if the message is multiline then add tag.

If the parsing of those multilines was from logstash I will use multliline_tag.

But how can I tag those multilines when the parsing is done in filebeat?