Include Praefect usage in the usage ping

With introduction of praefect between GitLab and Gitaly it is required to include info about Praefect into Usage Ping. As Praefect is a transparent proxy between two listed above the new parameter is added to the response and will be filled by the Praefect based on the Gitalies behind it. Later it will be used by GitLab to calculate the number of storages and report it by Usage Ping.

Part of: #2664 (closed)