Feature/metrics docs change

Documentation changes 

1) Gauge Metric Type support
2) Moved /history end-point to /runs, which includes currently running program and options to filter by status
3) Metrics Querying by run-id
Each execution of an element...has an associated run-ID that uniquely identifies that element’s run.
We can query metrics for an element by its run-ID to see the metrics for a particular run.
Please see the :ref:`Run Records and Schedule <rest-element-runs>` on retrieving active and historical element runs.