Handling KeyError "no buffer_queue_length key"

* All `output_plugin`s don't support output buffer information to `monitor_agent`'s json.

e.g: copy type plugin.. etc

```json
{
  "plugins": [
    {
      "config": {
        "type": "copy"
      },
      "output_plugin": true,
      "plugin_id": "object:3fbd2af24a64",
      "retry_count": null,
      "type": "copy"
    }
  ]
}
```
Merge pull request #2 from JumpeiArashi/handling-buffer-queue-length-error

Handling KeyError "no buffer_queue_length key"