Manages long-running operations with an API service.
When an API method normally takes long time to complete, it can be designed to return
Operation to the client, and the client can use this interface to receive the real response asynchronously by polling the operation resource, or pass the operation resource to another API (such as Google Cloud Pub/Sub API) to receive the response. Any API service that returns long-running operations should implement the
Operations interface so developers can have a consistent client experience.