[secor][progress-monitor] Add option to print metrics as JSON to stdout

This PR adds the ability to export metrics to stdout as JSON, rather than just to a remote tsdb server. This gives us the flexibility to do minor transforms to the JSON and send them to our collector of choice.
`ProgressMonitor` handles the null case gracefully. @pgarbacki: Would you like me to patch `PartitionFinalizer` to handle the null case?