Include total project snippet repository size in project statistics

At the moment, when a wiki or a project repository receives a push, we expire and recalculate some of the project statistics, in order to have, for example, an accurate storage size.

Now, with snippets having repositories, I think we should include this metric.

In #213526 (closed) we expire the snippet repository size. Therefore, when we call the ProjectCache worker, all the snippet repository sizes should be up to date. The task will be to sum al the snippet repo sizes and include them in the overall project storage size.

Basically, we can:

• Add a column in ProjectStatistics like snippets_size
• In https://gitlab.com/gitlab-org/gitlab/blob/master/app/workers/post_receive.rb#L75, add a call to call like ProjectCacheWorker.perform_async(project.id, [], [:snippets_size]) after we expire the snippet repository size cache.
• Update the ProjectStatistic model to include this new snippets_size metric
• We should also call the ProjectCacheWorker when we remove a snippet.
• Include the new metric in those views we show them (at the moment I'm only aware of the Admin section)

Test Activity

@fjsanpedro:

• Test that the statistics are updated properly when the repository is updated (more size/lower size)

@vij:

• Test statistic is incremented correctly
• Test statistic is decremented correctly
• Test statistic is included in admin views

MR Breakdown

@fjsanpedro:

We need to update one table in the database, the related class to include snippets, and also the places when we render this statistics, which can be views or API endpoints.

• 1 MR to add the column to the database
• 1 MR to update the main class
• 1 MR to update API endpoints
• 1 MR to update views