Closes #21043

GitLab Performance Monitoring is now able to track custom events not
directly related to application performance. These events include the
number of tags pushed, repositories created, builds registered, etc.

The use of these events is to get a better overview of how a GitLab
instance is used and how that may affect performance. For example, a
large number of Git pushes may have a negative impact on the underlying
storage engine.

Events are stored in the "events" measurement and are not prefixed with
"rails_" or "sidekiq_", this makes it easier to query events with the
same name triggered from different parts of the application. All events
being stored in the same measurement also makes it easier to downsample
data.

Currently the following events are tracked:

* Creating repositories
* Removing repositories
* Changing the default branch of a repository
* Pushing a new tag
* Removing an existing tag
* Pushing a commit (along with the branch being pushed to)
* Pushing a new branch
* Removing an existing branch
* Importing a repository (along with the URL we're importing)
* Forking a repository (along with the source/target path)
* CI builds registered (and when no build could be found)
* CI builds being updated
* Rails and Sidekiq exceptions

Fixes gitlab-org/gitlab-ce#13720

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4964#note_12744656

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4964#note_12741046

We store the sizes as a hash from path to bytes like:

``` ruby
{'ci_artifacts.txt' => 27,
 'other_artifacts_0.1.2/another-subdirectory/banana_sample.gif' =>
71759,
 'other_artifacts_0.1.2/doc_sample.txt' => 1314,
 'rails_sample.jpg' => 35255,
 'tests_encoding/utf8 test dir ✓/regular_file_2' => 7}
```

So that it's easier to access than reading gzip file again.

This reverts commit 13e37a3e.

Also add a test for setting locked.

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

- Fix naming of API::CommitStatuses
- Ensure we use require_dependency instead of require
- Ensure the namespace is right in lib/api/api.rb, otherwise, we
  might require Grape::API::Helpers which defines the `#params` method.
  This is to avoid requiring a file multiple times and getting an "Already
  initialized constant" error.
Signed-off-by: NRémy Coutable <remy@rymai.me>

This reverts commit 0a280158.

- So that the server doesn't have to be restarted for every change in dev.

- Makes the MR easier to read; this can go in a separate MR
- This is a (sort of) revert of 99bea01

[ci skip]

[ci skip]