[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.
If we make these private then json-smart will throw a fit when it tries to serialize them. It makes me sad too, but it won't work otherwise unless we switch JSON libs.