Add to prometheus metrics:

• number of cloud operations, split by operation type (op=Create/Destroy/List/SetTags) and result (error=0/1)
• can be implemented as a driver proxy similar to rateLimitedInstanceSet in source:lib/dispatchcloud/driver.go
• most likely usage in graphs/alerts is arvados_dispatchcloud_driver_operations{error=1}

• number of times an instance disappeared in cloud (see sync() in source:lib/dispatchcloud/worker/pool.go), split by state
• most likely usage in graphs/alerts is arvados_dispatchcloud_instances_disappeared{state!="shutdown"}