monitor_agent does strange output in v0.14

[kazegusuri]

monitor_agent's ouput in v0.14 is weird and different from in v0.12. With folloing configuration:

<source>
  @type monitor_agent
</source>

<match nginx>
  @type copy
  <store>
    @type stdout
  </store>
</match>

<match nginx2>
  @type stdout
</match>

in v0.12.29

$ curl http://localhost:24220/api/plugins
plugin_id:object:3fe8b81d9c10   plugin_category:input   type:monitor_agent  output_plugin:false retry_count:
plugin_id:object:3fe8b857d768   plugin_category:output  type:copy   output_plugin:true  retry_count:
plugin_id:object:3fe8b857c46c   plugin_category:output  type:stdout output_plugin:true  retry_count:
plugin_id:object:3fe8b81f4038   plugin_category:output  type:stdout output_plugin:true  retry_count:

in v0.14.6

$ curl http://localhost:24220/api/plugins
plugin_id:object:3fa09f9fad74   plugin_category:input   type:monitor_agent  output_plugin:false retry_count:
plugin_id:object:3fa09f53ae4c   plugin_category:unknown type:copy   output_plugin:false retry_count:0
plugin_id:object:3fa09f4ed32c   plugin_category:output  type:stdout output_plugin:true  buffer_total_queued_size:0  retry_count:0
plugin_id:object:3fa09f4ed32c   plugin_category:output  type:stdout output_plugin:true  buffer_total_queued_size:0  retry_count:0
plugin_id:object:3fa09fa5e5e0   plugin_category:output  type:stdout output_plugin:true  buffer_total_queued_size:0  retry_count:0

There seems to be 3 issues.

1. copy plugin is categorized as unknown
2. stdout plugin inside copy plugin is duplicate
3. stdout plugin is not buffered output plugin but displays metrics for buffered plugin such as buffered_total_queue_size

[tagomoris]

• copy plugin is categorized as unknown.

It's the bug of #plugin_category, not to consider about Fluent::Plugin::MultiOutput.

[tagomoris]

• stdout plugin inside copy plugin is duplicated

It's the bug that in_monitor_agent plugin fetches output plugin instances from root_agent.outputs, and output_instance.outputs if output plugin is an instance of MultiOutput.
Now root_agent.outputs contains all output plugin instances including children of MultiOutput.

[tagomoris]

• stdout plugin is not buffered output plugin but displays metrics for buffered plugin such as buffered_total_queue_size

This bug description is not correct, because out_stdout can work as both of buffered and non-buffered plugin now (v0.14.5 and later).
But we can remove the buffer instance from @buffer when it works as non-buffered output. If we do so, we can update code of in_monitor_agent, and buffer_total_queued_size will disappear.