title: is there way to set timestamp in unbounded source pcollection?
body: <p>i want to set timestamp to the unbounded pcollection of strings </p>
  
  <p>in my solution each line of the pcollection is a row of csv </p>
  
  <p>in one field of this row have a timestamp and others fields like number of clicks etc. </p>
  
  <p>i want to process the collection with base on its own timestamp (event time )not the default timestamp that set apache beam</p>
  
  <p>the main threat of this dataflow is group the number of clicks per minute </p>
  
  <p>i have a ftp service that send files to one folder that my job listen each 1 min and process in fix window of 1 min 
  the problem is that is possible arrive late data</p>
  
  <p>i do KV pcolection and key is the timestamp without seconds and value num of clicks and apply group by </p>
  
  <p>i get "clicks per minute" and i send this pcollection to database </p>
  
  <p>example 
  package of data element generate to 12:05pm (10 files)
  time recive the data 12:06pm </p>
  
  <p>Job generation data
  12:05pm 120 clicks </p>
  
  <p>package of data element generate to 12:05pm  and 12:06 (files late of 12:05)</p>
  
  <p>time receive the data 12:07pm </p>
  
  <p>Job generation data 
  12:05pm 10 clicks
  12:06pm 135 clicks</p>
  
  <p>i want discard the process of the late data based in event data 
  is possible in the way that i am search</p>
  
  <p>in other words dont put in db the late data -> 12:05pm  -10clicks</p>