title: fluentd nested json parsing
body: <p>I have logs like the following:</p>
  
  <pre><code>{
    "log": {
      "header": {
        "key": "value",
        "nested": "{\"key1\":\"value\",\"key2\":\"value\"}",
        "dateTime": "2019-05-08T20:58:06+00:00"
      },
      "body": {
        "path": "/request/path/",
        "method": "POST",
        "ua": "curl/7.54.0",
        "resp": 200
      }
    }
  }
  </code></pre>
  
  <p>I'm trying to aggregate logs using fluentd and I want the entire record to be JSON. The specific problem is the "$.log.header.nested" field, which is a JSON string. How can I parse and replace that string with its contents?</p>
  
  <p>For clarity, I'd like the logs output by fluentd to look like this:</p>
  
  <pre><code>{
    "log": {
      "header": {
        "key": "value",
        "nested": {
            "key1": "value",
            "key2": "value"
        },
        "dateTime": "2019-05-08T20:58:06+00:00"
      },
      "body": {
        "path": "/request/path/",
        "method": "POST",
        "ua": "curl/7.54.0",
        "resp": 200
      }
    }
  }
  </code></pre>
  
  <p>I've found a way to parse the nested field as JSON, but storing to back to the same key it was parsed from isn't clear. It doesn't seem like <code>hash_value_field</code> supports storing to a nested key. Is there some other way to accomplish this?</p>