Service discovery

Consider a scenario where application instances are dynamically started and publish their endpoints, while other clients need to receive this list and respond to its changes.

This scenario can be implemented using semaphores in YDB coordination nodes as follows:

1. Create a semaphore (for example, named my-service-endpoints) with Limit=Max<ui64>().
2. All application instances call AcquireSemaphore with Count=1, specifying their endpoint in the Data field.
3. Since the semaphore limit is very high, all AcquireSemaphore calls should complete quickly.
4. At this point, publication is complete, and application instances only need to respond to session stops by republishing themselves through a new session.
5. Clients call DescribeSemaphore with IncludeOwners=true and optionally with WatchOwners=true. In the result, the Owners field's Data will contain the endpoints of registered application instances.
6. When the list of endpoints changes, OnChanged is called. In this case, clients make a similar DescribeSemaphore call and receive the updated list.