* We realized that headers were not set whenever we give 204
  because `render_api_error!` doesn't preserve the headers.

* We also realized that `update_runner_info` would be called in
  POST /builds/register every time therefore runner is updated
  every time, ticking the queue, making this last_update didn't
  work very well, and the test would be failing due to that.

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8091#note_20222666

Closes #25309

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.

[ci skip]

[ci skip]

Also added API specs for that

Due to broken implementation of attribute_for_keys the runner information was not updated correctly.

This MR adds test to check that such scenario will never happen again.

This allows us to implement artifacts passing: runner will download artifacts from all prior builds

We do not want to allow runners to upload a metadata file. This needs to
be generated by Workhorse only.