export prometheus metrics

[olevski]

We should export some simple prometheus metrics from the operator:

• session launches (should be a counter), add authentication type as a single metric label (this way we can distinguish anonymous and registered sessions)
• session deletions (should be a counter) same labels as the launches counter above
• existing sessions (should be a gauge), separate by labels for status (Ready, Not Ready, Failing, Running) and idleness
• hibernated sessions (should be a gauge)

We can use the golang prometheus library. https://github.com/prometheus/client_golang

It would be nice if the metrics code is a bit isolated into its own package.