in_monitor_agent emits plugins info to use exisiting plugins. fix #667

[repeatedly]

Here is test logs.

% RUBYLIB=~/dev/fluentd/fluentd/lib fluentd -v -c monitor_agent.conf                                                                        (git)-[master]
2015-09-04 21:27:41 +0900 [info]: fluent/supervisor.rb:403:read_config: reading config file path="monitor_agent.conf"
2015-09-04 21:27:41 +0900 [info]: fluent/supervisor.rb:279:supervise: starting fluentd-0.12.15
2015-09-04 21:27:41 +0900 [info]: fluent/engine.rb:100:block in configure: gem 'fluent-mixin-config-placeholders' version '0.3.0'
2015-09-04 21:27:41 +0900 [info]: fluent/engine.rb:100:block in configure: gem 'fluent-plugin-s3' version '0.5.10'
2015-09-04 21:27:41 +0900 [info]: fluent/engine.rb:100:block in configure: gem 'fluent-plugin-secure-forward' version '0.3.2'
2015-09-04 21:27:41 +0900 [info]: fluent/engine.rb:100:block in configure: gem 'fluent-plugin-td' version '0.10.27'
2015-09-04 21:27:41 +0900 [info]: fluent/engine.rb:100:block in configure: gem 'fluent-plugin-td-monitoring' version '0.2.0'
2015-09-04 21:27:41 +0900 [info]: fluent/engine.rb:100:block in configure: gem 'fluentd' version '0.12.12'
2015-09-04 21:27:41 +0900 [info]: fluent/agent.rb:123:add_match: adding match pattern="monitor.**" type="stdout"
2015-09-04 21:27:41 +0900 [info]: fluent/root_agent.rb:142:add_source: adding source type="monitor_agent"
2015-09-04 21:27:41 +0900 [info]: fluent/engine.rb:107:configure: using configuration file: <ROOT>
  <source>
    type monitor_agent
    tag monitor.test
    emit_interval 2
  </source>
  <match monitor.**>
    type stdout
  </match>
</ROOT>
2015-09-04 21:27:41 +0900 [debug]: plugin/in_monitor_agent.rb:235:start: listening monitoring http server on http://0.0.0.0:24220/api/plugins
2015-09-04 21:27:41 +0900 [debug]: plugin/in_monitor_agent.rb:250:start: tag parameter is specified. Emit plugins info to monitor.test
2015-09-16 20:28:17 +0900 [debug]: plugin/in_monitor_agent.rb:250:start: tag parameter is specified. Emit plugins info to 'monitor.test'
2015-09-16 20:28:19 +0900 monitor.test: {"plugin_id":"object:3fc62f0e5d64","plugin_category":"input","type":"monitor_agent","output_plugin":false,"retry_count":null}
2015-09-16 20:28:19 +0900 monitor.test: {"plugin_id":"object:3fc62f0e9c84","plugin_category":"output","type":"stdout","output_plugin":true,"retry_count":null}

[tatsu-yam]

👍