How to tag events with a tag?

Question

Monitoring

Actually the question is how to mark events with a tag? in fluentd?
here is an example of my config

<source>
  @type tail
  path /var/log/nginx/access.log
  pos_file /var/log/td-agent/nginx-access.log.pos
  tag nginx.access
  format nginx
</source>

<source>
  @type tail
  path /var/log/nginx/error.log
  pos_file /var/log/td-agent/nginx-error.log.pos
  tag nginx.access
  format nginx
</source>

<match nginx.access>
  @type elasticsearch
  logstash_format true
  host 127.0.0.1
  port 9200
  index_name apache.access
  type_name nginx
  flish_interval 5s
</match>

here is an example on habré

К стандартным плагинам вывода относятся match и forward:
#  Получаем события с порта 24224
<source>
  type forward
  port 24224
</source>

# http://this.host:9880/myapp.access?json={"event":"data"}
<source>
  type http
  port 9880
</source>

#Берём события, помеченные тэгами "myapp.access" 
#и сохраняем их в файле/var/log/fluent/access.%Y-%m-%d
#данные можно разбивать на порции с помощью опции time_slice_format.

<match myapp.access>
  type file
  path /var/log/fluent/access
</match>

http://this.host:9880/myapp.access?json={ "event":"data"} What is this string and where does it come from?