[Feature Proposal] - GameServer status hook for counter changes

Is your feature request related to a problem? Please describe. The new counter feature allows us to pack multiple game sessions to one game server. However, it's still not possible to attach the game server's status to the counter atomically. For instance, a game server with max capacity of 8 may want to be marked as "allocated" when there's 1-8 sessions on it and "ready" when there's 0 session such that the game server can be recycled when there's a new deployment, but currently the second part is not supported by the Agones framework and hard to do without race conditions.

Describe the solution you'd like Some sort of hook that ties the counter value to the status of the game server.

Describe alternatives you've considered Manually adjusted the server status right before/after modifying the counter value using the SDK, but it introduces race conditions.

Additional context None.

Link to the Agones Feature Proposal (if any) N/A.

Discussion Link (if any) N/A.