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 metrics
5. Kernel start / stop latency metrics
6. 0mq metrics

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