title: Cannot group by timestamp_trunc
body: <p>I have this query in standard sql.</p>
  
  <pre><code>select timestamp_trunc(endTime, MONTH), count(1)
  from `simple_table`
  group by timestamp_trunc(endTime, MONTH);
  </code></pre>
  
  <p>Which returnts the following error:</p>
  
  <p>SELECT list expression references column endTime which is neither grouped nor aggregated at [1:24]</p>
  
  <p>However, the following code:</p>
  
  <pre><code>select timestamp_trunc(endTime, MONTH)
  from `simple_table`
  limit 10
  </code></pre>
  
  <p>Works perfectly.  Is there some hidden reference about BigQuery's ability to do group by that I am missing?</p>