A feature to query the actual resource allocation time for compute sessions

Compute sessions have three status-related time fields: `created_at`, `status_changed`, and `terminated_at`. With these fields, we can query the total session lifetime, the time between `PENDING` and `TERMINATED` statuses, but unable to get the actual resource allocation time (`PREPARING` ~ `TERMINATED`). If a session stays long in `PENDING` status, the actual resource allocation time can greatly differ compared to the result of `terminated_at - created_at`.

Since the actual resource allocation time is useful to calculate the billing metric in cloud and on-premise sites, it would be good to support this feature.