[Step 4] Update namespace statistics after snippets update
At the moment, when a snippet is updated, only the project statistics get updated, not the namespace ones. This is because we didn't include the
snippets_size statistics inside here. That would make that any change in the
snippets_size triggers an update in the namespace statistics.
We didn't do this at first by default in order not to hurt performance, the discussion is ec39a1d6 (comment 367932364).
Nevertheless, we can't use the other resource we have because snippet repositories can be updated directly through git, therefore no callback in any ActiveRecord model will be called. Besides, with the snippet we can update the title, for example, and that shouldn't trigger and update.
In this issue, we'll introduce the feature flag
namespace_snippets_size_stat to include the
snippets_size inside https://gitlab.com/gitlab-org/gitlab/blob/master/app/models/project_statistics.rb#L16 and trigger an automatic update.