[XDCR] No XDCR Metrics for Status of a Replication

To effectively implement alerting around XDCR the status of a replication needs to be known.  The ns_server REST APIs return a status of "running", "paused", "error".  If this value is exposed, I can create an alert on whether or not the replication has been paused as an example a customer may wish to alert on the fact a replication has been paused for more than 1 days time.  

Prometheus metrics must be a number:

xdcr_status{status="running"} 1

xdcr_status{status="paused"} 0
xdcr_status{status="error"} 0

or

xdcr_status{status="running"} 0

xdcr_status{status="paused"} 1
xdcr_status{status="error"} 2