Add more Prometheus metrics

Now that #3490 has been merged, we should add more prometheus metrics to the notebook server!

Some ideas for metrics to add...

1. Number of kernels running (labeled by type)
2. Number of sessions open (not sure if this is useful?)
3. Number of terminals open
4. Mirror of the [activity tracking](https://github.com/jupyter/notebook/issues/1241) metrics
5. Kernel start / stop latency metrics
6. 0mq metrics

Am sure there's more that I don't know of!

Start by reviewing the metrics work in #3490 and the activity-tracking discussion in #1241. Narrow the proposed list into a decided set of Prometheus metrics for the notebook server, then confirm that each selected metric has an implementation and corresponding verification before considering the issue done.

Written by the indexing model from the issue text.