Implement gRPC health check protocol

It [1] would allow to use it in the future kubernetes deployment [2].

[1] https://github.com/grpc/grpc/blob/master/doc/health-checking.md

[2] https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/#define-a-grpc-liveness-probe

marked this issue as related to swh/infra/sysadm-environment#5382 (closed)

Should it be considered healthy when all workers are busy?

nvm, it would just not be able to answer such requests, so it would be unhealthy

mentioned in merge request !647 (merged)

closed with merge request !647 (merged)