Add admin documentation for gitaly-ruby

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diff --git a/doc/administration/gitaly/index.md b/doc/administration/gitaly/index.md
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+++ b/doc/administration/gitaly/index.md
@@ -315,6 +315,66 @@ certificate_path = '/path/to/cert.pem'
 key_path = '/path/to/key.pem'
 ```
 
+## Gitaly-ruby
+
+Gitaly was developed to replace Ruby application code in gitlab-ce/ee.
+In order to save time and/or avoid the risk of rewriting existing
+application logic, in some cases we chose to copy some application code
+from gitlab-ce into Gitaly almost as-is. To be able to run that code, we
+made gitaly-ruby, which is a sidecar process for the main Gitaly Go
+process. Some examples of things that are implemented in gitaly-ruby are
+RPC's that deal with wiki's, and RPC's that create commits on behalf of
+a user, such as merge commits.
+
+### Number of gitaly-ruby workers
+
+Gitaly-ruby has much less capacity than Gitaly itself. If your Gitaly
+server has to handle a lot of request, the default setting of having
+just 1 active gitaly-ruby sidecar might not be enough. If you see
+ResourceExhausted errors from Gitaly it's very likely that you have not
+enough gitaly-ruby capacity.
+
+You can increase the number of gitaly-ruby processes on your Gitaly
+server with the following settings.
+
+Omnibus:
+
+```ruby
+# /etc/gitlab/gitlab.rb
+# Default is 2 workers. The minimum is 2; 1 worker is always reserved as
+# a passive stand-by.
+gitaly['ruby_num_workers'] = 4
+```
+
+Source:
+
+```toml
+# /home/git/gitaly/config.toml
+[gitaly-ruby]
+num_workers = 4
+```
+
+### Observing gitaly-ruby traffic
+
+Gitaly-ruby is a somewhat hidden, internal implementation detail of
+Gitaly. There is not that much visibility into what goes on inside
+gitaly-ruby processes.
+
+If you have Prometheus set up to scrape your Gitaly process, you can see
+request rates and error codes for individual RPC's in gitaly-ruby by
+querying `grpc_client_handled_total`. Strictly speaking this metric does
+not differentiate between gitaly-ruby and other RPC's, but in practice
+(as of GitLab 11.9), all gRPC calls made by Gitaly itself are internal
+calls from the main Gitaly process to one of its gitaly-ruby sidecars.
+
+Assuming your `grpc_client_handled_total` counter only observes Gitaly,
+the following query shows you RPC's are (most likely) internally
+implemented as calls to gitaly-ruby.
+
+```
+sum(rate(grpc_client_handled_total[5m])) by (grpc_method) > 0
+```
+
 ## Disabling or enabling the Gitaly service in a cluster environment
 
 If you are running Gitaly [as a remote