[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 make this a subclass of JSONObject and use .put() rather than relying on reflection. I feel that that doing that would also drastically simplify the code - you won't need the private fields or custom getters.