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
Perhaps we need to add something about the different types of metrics now available. Do they show up in the CDAP Console in different ways? What are they intended for? How many types are there now? Just two, count and gauge?