Add GetStorageDiskStatistics grpc

It may also be useful to implement a method for obtaining a collection of disk space statistics. In fact, this could reduce overall load on the server, by avoiding the need to invoke multiple remote procedure calls to obtain separate details.

So this issue is to suggest the feature addition of a grpc resembling GetStorageDiskStatistics which would return a collection, ideally a dictionary, containing at least the disk avail and disk used portions of the output of a df /var/opt/gitlab invocation on the given target storage node system.