Track Mattermost usage

doc/user/admin_area/settings/usage_statistics.md

@@ -75,6 +75,9 @@ The total number of the following is sent back to GitLab Inc.:
 - Remote mirrors
 - Web hooks
 
+Also, we track if you've installed Mattermost with GitLab. 
+For example: `"mattermost_enabled":true"`.
+
 ## Privacy policy
 
 GitLab Inc. does **not** collect any sensitive information, like project names

lib/gitlab/usage_data.rb

@@ -53,7 +53,9 @@ module Gitlab
 
       def license_usage_data
         usage_data = { version: Gitlab::VERSION,
-                       active_user_count: User.active.count }
+                       active_user_count: User.active.count,
+                       mattermost_enabled: Gitlab.config.mattermost.enabled }
+
         license = ::License.current
 
         if license

spec/lib/gitlab/usage_data_spec.rb

@@ -21,6 +21,7 @@ describe Gitlab::UsageData do
         license_md5
         recorded_at
         version
+        mattermost_enabled
       ))
     end