I have a use case where I would like to start publishing messages to a
PubSub topic in advance of starting a streaming pipeline that consumes
those messages. When the pipeline starts I would like it to "catch up"
which is to say to read all of the messages that were published even before
it started. In this case event time doesn't matter to me. All that matters
is that I see every message once.
My initial thought was something like this:
```
trg := trigger.AfterAny([]trigger.Trigger{
trigger.Repeat(trigger.AfterProcessingTime().PlusDelay(time.Minute * 1)),
trigger.AfterCount(1_000),
})
beam.WindowInto(s, window.NewGlobalWindows(), x, beam.Trigger(trg),
beam.PanesDiscard())
```
Basically process a minutes worth OR a 1000 messages. This definitely
doesn't work though. Later on I call `databaseio.Write`, and that never
does anything. However if I publish a message _while_ the pipeline is
running `databaseio.Write` seems to pick it up. I will freely admit I don't
have much of an idea what I'm doing here.
