[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.
I think we should just throw an exception. To this end, I'm not sure if we even need to modify this piece of code. I think the old logic was doing exactly that.