title: Dataflow GroupByKey transform splits input rows
body: <p>I run a data flow job to read data from files stored in GCS, each record has an "event type", my goal is to split the data per "event type" and write each output to a bq table, now I'm using a filter to do this, however I'd like to try GroupByKey transform which hopefully can make the process dynamic as new Event Types will flow in over time which can't be predicted at the development time. So now my challenge is, I don't know if its possible to construct a WRITE transform per each KEY(the key from output GroupByKey)?  It would be ideal if its doable, or any other ways can achieve this, any advice would be appreciated</p>