title: Converting selected JSON tags to Pandas dataframe
body: <p>I have a JSON like - </p>
  
  <pre><code>[
    {
      "key": "033298fd-f792-4343-b145-852e9cdb680a",
      "value": {
        "total": 15452,
        "history": {
          "2019-11-05T23:05:14.53878Z": {
            "challenge": "readrules",
            "increase": 1,
            "total": 1
          },
          "2019-11-05T23:17:48.849886Z": {
            "challenge": "looksthesame",
            "increase": 100,
            "total": 1601
          }
       },
    ...
    ...
    ...
    }
  
   .... 2nd 'key' ....
  }
  
  </code></pre>
  
  <p>where every 'key' is followed by a 'value' which has <code>history</code>. This 'history' again has a 'key' which is the <code>timestamp</code> and a value that has challenge details <code>challenge</code>, <code>increase</code> and <code>total</code>. 
  I want to convert this to a pandas dataframe that looks something like - </p>
  
  <pre><code>key                                              timestamp                   challenge       increase   total
  033298fd-f792-4343-b145-852e9cdb680a    2019-11-05T23:05:14.53878Z      readrules          1         1 
  033298fd-f792-4343-b145-852e9cdb680a    2019-11-05T23:17:48.84986Z      looksthesame      100       101
  </code></pre>
  
  <p>I tried to work with - </p>
  
  <pre><code>pd.io.json.json_normalize(json)
  </code></pre>
  
  <p>but that just flattens out the whole json into different columns.  </p>