Add step duration as an indexed field

[andrewvc]

We currently index the step duration of the field on the payload object. Users cannot search this data or make custom kibana visualizations with it. It would be useful to make step timing charts not only in uptime, but also in kibana dashboards.

This issue proposes we create a new field synthetics.step.duration.us which measures the step execution time in milliseconds. We can use the root_fields JSON to add this field.

I had discussed with @vigneshshanmugam the possibility of making a new field for journey durations as well, however the existing monitor.duration.us field covers this. I'd rather not introduce a second field with the same information in milliseconds at this point since that may be confusing.

[vigneshshanmugam]

@andrewvc To make use of the monitor.duration.us, Do you think we can send the timestamp field denoting the proper journey end time as opposed to when the JSON output journey/end is written which might get fluctuated?