Monolog Logstash Config example

logstash.conf

input {
  stdin { codec => "plain" }
}

filter {
  # Pulls out fields from monolog text log. (Note, we don't send extra to our monolog)
  grok {
    match => [ "message", "%{MONOLOG} %{GREEDYDATA:mymessage}"]
  }

  json {
    source => "context"
  }
}

output {
  stdout { codec => rubydebug }
}

You need to add overwrite => [ "message" ] to set properly the message field.