[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.
You could simplify this a tiny bit by saying:
```
if (!mesageSetIearator.hasNext()) {
  return null;
}
MessageAndOffset messageAndOffset = messageSetIterator.next();
...
```