title: Convert from PCollection<TableRow> to PCollection<KV<K,V>>
body: <p>I'm trying to extract data from 2 tables in BigQuery, then join it by CoGroupByKey.
  Although the output of BigQuery is <code>PCollection&lt;TableRow&gt;</code>, <code>CoGroupByKey</code> requires <code>PCollection&lt;KV&lt;K,V&gt;&gt;</code>.
  How can I convert from <code>PCollection&lt;TableRow&gt;</code> to <code>PCollection&lt;KV&lt;K,V&gt;&gt;</code>?</p>