Provide a way for user code to read dataflow runner stats

The dataflow runner collects (and publishes to the dataflow service) a large number of useful stats.  While these can be polled from the dataflow service via its API, there are a few downsides to this:
* it requires another process to poll and collect the stats
* the stats are aggregated across all workers, so per-worker stats are lost

It would be simple to provide a hook to allow users to receive stats updates as well, and then do whatever they want with them.

Imported from Jira [BEAM-7605](https://issues.apache.org/jira/browse/BEAM-7605). Original Jira may contain additional context.
Reported by: SteveNiemitz.