[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.
Sorry for the late followup, but are you saying you want `Stat#toString()` to return a JSON string representation of the stat instance and just using that to feed into tsdb?