Ignore log_level parameter in config

[ingtarius]

td-agent 1.2.6, build from sources 3.2.1
Add section to config
<system> log_level error <log> format json time_format %Y-%m-%d_%H:%M:%S </log> </system>
but see warnings in log
{"time":"2018-11-15_14:25:47","level":"warn","worker_id":0,"message":"suppressed same stacktrace"}