BTW, I tested parser plugin and it worked with your log example.
2016-10-27 04:36:43 +0900 [info]: reading config file path="postgre_parser.conf"
2016-10-27 04:36:43 +0900 [info]: starting fluentd-0.12.29
2016-10-27 04:36:43 +0900 [info]: gem 'fluent-plugin-beats' version '0.1.1'
2016-10-27 04:36:43 +0900 [info]: gem 'fluent-plugin-elasticsearch' version '1.7.0'
2016-10-27 04:36:43 +0900 [info]: gem 'fluent-plugin-flowcounter-simple' version '0.0.4'
2016-10-27 04:36:43 +0900 [info]: gem 'fluent-plugin-parser' version '0.6.1'
2016-10-27 04:36:43 +0900 [info]: gem 'fluent-plugin-record-modifier' version '0.5.0'
2016-10-27 04:36:43 +0900 [info]: gem 'fluent-plugin-s3' version '0.7.1'
2016-10-27 04:36:43 +0900 [info]: gem 'fluent-plugin-secure-forward' version '0.4.2'
2016-10-27 04:36:43 +0900 [info]: gem 'fluent-plugin-td' version '0.10.29'
2016-10-27 04:36:43 +0900 [info]: gem 'fluentd' version '0.12.29'
2016-10-27 04:36:43 +0900 [info]: adding filter pattern="debug.*" type="parser"
2016-10-27 04:36:43 +0900 [info]: adding match pattern="debug.*" type="stdout"
2016-10-27 04:36:43 +0900 [info]: adding source type="forward"
2016-10-27 04:36:43 +0900 [info]: using configuration file: <ROOT>
<source>
@type forward
</source>
<filter debug.*>
@type parser
format json
key_name sessions
</filter>
<match debug.*>
@type stdout
</match>
</ROOT>
2016-10-27 04:36:43 +0900 [info]: listening fluent socket on
0.0.0.0:242242016-10-27 04:37:07 +0900 debug.test: {"2017-10-26T08:03:13.116+00:00":"open","2017-09-10T17:03:13.116+00:00":"open"}
^C2016-10-27 04:37:16 +0900 [info]: shutting down fluentd
2016-10-27 04:37:16 +0900 [info]: shutting down input type="forward" plugin_id="object:3ff8c6c8c504"
2016-10-27 04:37:16 +0900 [info]: shutting down filter type="parser" plugin_id="object:3ff8c600cf68"
2016-10-27 04:37:16 +0900 [info]: shutting down output type="stdout" plugin_id="object:3ff8c6c11048"
2016-10-27 04:37:16 +0900 [info]: process finished code=0