Unfortunately, this hasn't been implemented, while it had been asked for several times. See
https://github.com/grpc/grpc-java/issues/1886 for status and progresses about the discussion. You could comment (ping) on that with descriptions about your use case. There may be other server concurrency limiting solutions mentioned by other users in that discussion interests you.
FWIW, our service implementations used to have load reporting mechanisms that servers report their load (e.g., # of requests, utilizations, etc) to the balancer for load balancing purposes.