title: Apache Beam TextIO.Read with line number
body: <p>Is it possible to get access to line numbers with the lines read into the PCollection from TextIO.Read? For context here, I'm processing a CSV file and need access to the line number for a given line.</p>
  
  <p>If not possible through TextIO.Read it seems like it should be possible using some kind of custom Read or transform, but I'm having trouble figuring out where to begin.</p>