title: count after groupby in Google Dataflow
body: <p>I have the following in my Google cloud storage</p>
  
  <pre><code>Advertiser | Event
  __________________
  100 | Click
  
  101 | Impression
  
  100 | Impression
  
  100 | Impression
  
  101 | Impression
  </code></pre>
  
  <p>My output of the pipeline should be something like</p>
  
  <pre><code>Advertiser | Count
  
  100 | 3
  
  101 | 2
  </code></pre>
  
  <p>First I used groupByKey, the output is like</p>
  
  <pre><code>100 Click, Impression, Impression
  
  101 Impression, Impression
  </code></pre>
  
  <p>How to proceed from here?</p>