title: Pandas groupby agg - how to get counts?
body: <p>I am trying to get sum, mean and count of a metric</p>
  
  <pre><code>df.groupby(['id', 'pushid']).agg({"sess_length": [ np.sum, np.mean, np.count]})
  </code></pre>
  
  <p>But I get "module 'numpy' has no attribute 'count'", and I have tried different ways of expressing the count function but can't get it to work. How do I just an aggregate record count together with the other metrics?</p>