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

@@ -47,7 +47,8 @@ module Gitlab
       def license_usage_data
         usage_data = { version: Gitlab::VERSION,
                        active_user_count: User.active.count,
-                       recorded_at: Time.now }
+                       recorded_at: Time.now,
+                       mattermost_enabled: Gitlab.config.mattermost.enabled }
         usage_data
       end

spec/lib/gitlab/usage_data_spec.rb

@@ -14,6 +14,7 @@ describe Gitlab::UsageData do
         counts
         version
         recorded_at
+        mattermost_enabled
       ))
     end