Unblock tunnel stream when incoming stream context is done

The symptom that I observed is that there was grpc_server_requests_in_flight{grpc_method="MakeRequest",grpc_service="gitlab.agent.kubernetes_api.rpc.KubernetesApi"} 1 metric (notice 1 instead of zero) even after I terminated kubectl call.

Found while investigating gitlab-org/gitlab#343148 (closed), but it's not the cause of it.