[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.
Let's try to minimize the number of configuration params. To this end, let's always print out the json metrics to stdout and post to tsdb only if tsdb.hostport is set. Then this param can be removed altogether.