title: Pandas: how to transform complex dataframe to simpler dataframe
body: <p>I have below raw data already in a dataframe</p>
  
  <pre><code>{'timePeriod': {'start': '2019-09-01', 'end': '2019-09-02'},
    [{'keys': ['173419835872', 'Software'],
      'metrics': {'Cost': {'amount': '1100.23', 'unit': 'USD'}}},
     {'keys': ['921747472697', 'Hardware'],
      'metrics': {'Cost': {'amount': '2300.11', 'unit': 'USD'}}},
     {'keys': ['921747472697', 'Tax'],
      'metrics': {'Cost': {'amount': '500.00', 'unit': 'USD'}}}]
    }
  </code></pre>
  
  <p>I want to end up with a dataframe with five columns labeled: </p>
  
  <pre><code>year month acct type amt
  2019 Sep 173419835872 Software 1100.23
  2019 Sep 921747472697 Hardware 2300.11
  2019 Sep 921747472697 Tax 500.00
  </code></pre>
  
  <p>What df functions can I use to parse the raw data and create the new dataframe?</p>
  
  <p>Thank you.</p>