api/v3rpc: log grpc stream send/recv errors in server-side

[gyuho]

Helpful for debugging #8904.

c.f. grpc/grpc-go#1687

[jpbetz]

lgtm

Out of curiosity. When do we prefer emitting a warning like this versus expanding the error with more information, e.g. return fmt.Errorf("failed to recieve...: %s", err) ?

[gyuho]

@jpbetz

pb.Watch_WatchServer.Send/Recv is generated code. And https://github.com/coreos/etcd/blob/b21180d198f6c87d711980686af388f71c665126/etcdserver/api/v3rpc/watch.go#L111 is only called via https://github.com/coreos/etcd/blob/b21180d198f6c87d711980686af388f71c665126/etcdserver/etcdserverpb/rpc.pb.go#L3479-L3481

Those errors are being returned to gRPC-side, so we can only see those errors when gRPC error log is enabled--now disabled by default (#8810), since it was getting too verbose with other gRPC internal logging.

This etcd-side log will help us verify the fix proposed in grpc/grpc-go#1687.

I will also backport this to 3.2 as well.