Add documentation for gitaly_ruby_json.log

doc/administration/logs.md

@@ -582,6 +582,12 @@ This file lives in `/var/log/gitlab/gitaly/current` and is produced by [runit](h
 
 This file lives in `/var/log/gitlab/gitlab-rails/grpc.log` for Omnibus GitLab packages. Native [gRPC](https://grpc.io/) logging used by Gitaly.
 
+### `gitaly_ruby_json.log`
+
+> [Introduced](https://gitlab.com/gitlab-org/gitaly/-/merge_requests/2678) in GitLab 13.6.
+
+This file lives in `/var/log/gitlab/gitaly/gitaly_ruby_json.log` and is produced by [`gitaly-ruby`](gitaly/reference.md#gitaly-ruby). It contains an access log of gRPC calls made by Gitaly to `gitaly-ruby`.
+
 ## Puma Logs
 
 ### `puma_stdout.log`