use typecast

bq_schema.json

[
  {
    "name": "boolean_value1",
    "type": "BOOLEAN"
  },
  {
    "name": "boolean_value2",
    "type": "BOOLEAN"
  }
]

docker-compose.yml

version: '2'
services:
  fluentd:
    build: .
    container_name: fluentd
    volumes:
      - ./:/fluentd/etc/

Dockerfile

FROM fluent/fluentd:v0.12.26-onbuild

USER root
RUN apk --update add build-base ruby-dev
USER fluent
RUN gem install \
  fluent-plugin-bigquery \
  fluent-plugin-typecast \
  fluent-plugin-parser &&\
  fluent-gem install bigdecimal

fluent.conf

<source>
  @type dummy
  dummy {"message":"boolean_value1:true\tboolean_value2:false"}
  rate 1
  tag bigquery
</source>

<filter bigquery>
  @type parser
  format ltsv
  key_name message
</filter>

<match bigquery>
  @type typecast
  item_types boolean_value1:bool,boolean_value2:bool
  prefix typed
</match>

<match typed.bigquery>
  @type bigquery
  method insert
  auth_method private_key
  email xxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxxx@developer.gserviceaccount.com
  private_key_path 00000000000000000000000000000000-privatekey.p12
  project yukiyan-development
  dataset sample
  table   sample
  auto_create_table true
  schema_path /fluentd/etc/bq_schema.json
</match>

apk --update add build-base ruby-dev とか fluent-gem install bigdecimal はコンテナでfluent-plugin-bigquery動かす上で必要なやつなのであまり気にしなくて良い。