Adding metric tracking number of jobs running on a cluster

We frequently come across issues where in there are considerable number of jobs/notebooks running on one interactive cluster. This leads to either degraded performance due to cluster being overloaded or leads to driver OOM. It would be helpful to have a metric identifying number of notebooks/jobs running on a cluster as this would be important datapoint for customers to balance out their workloads.

We propose to add a metric which keeps track of number of jobs running on a cluster.