Signed-off-by: NTakuya Noguchi <takninnovationresearch@gmail.com>

Markdown renderers find it easier to determine
where lists start and end when lists are surrounded
by new lines.

For consistency, also ensure entries in the list
are aligned when they span multipls lines.

Update the list of GitLab metrics to include the missing metric names.

Signed-off-by: NTakuya Noguchi <takninnovationresearch@gmail.com>

Now the files are identical again compared to EE.

These are backported from
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5050

Was introduced in the time that GitLab still used NFS, which is not
required anymore in most cases. By removing this, the API it calls will
return empty responses. This interface has to be removed in the next
major release, expected to be 12.0.

* Update metric names to match current release.
* Improve descriptions.
* Improve examples to show more query language functionality.

Memory:
* Show use of `or` function.
* Automatically select "MemAvailable" kernel metric if possible.

CPU:
* Show use of `avg without ...`.
* Show CPU utilization per node.

Network:
* Show use of `!=` label matching.
* Don't show "localhost" metrics.

The pngquant tool was used https://pngquant.org, and particularly, the
following command:

        /usr/bin/pngquant -f --skip-if-larger --ext .png --speed 1 image.png

Before: 47584K
After : 34924K

* Use a simple counter for sampler duration instead of a histogram.
* Use a counter to collect GC time.
* Remove unused objects metric.
* Cleanup metric names to match Prometheus conventions.
* Prefix generic GC stats with `gc_stat`.
* Include worker label on memory and file descriptor metrics.

Update index.md to describe leaving out the host in prometheus['listen_address'] to allow public access.
Signed-off-by: NRémy Coutable <remy@rymai.me>

But first, rewrite the performance bar in Vue:

1. Remove the peek-host gem and replace it with existing code. This also allows
   us to include the host in the JSON response, rather than in the page HTML.
2. Leave the line profiler parts as here-be-dragons: nicer would be a separate
   endpoint for these, so we could use them on Ajax requests too.
3. The performance bar is too fiddly to rewrite right now, so apply the same
   logic to that.

Then, add features! All requests made through Axios are able to be tracked. To
keep a lid on memory usage, only the first two requests for a given URL are
tracked, though. Each request that's tracked has the same data as the initial
page load, with the exception of the performance bar and the line profiler, as
explained above.

This is as important as SQL timings, and much more important most of the time
than GC, Redis, or Sidekiq.

Prior to this MR there were two GitHub related importers:

* Github::Import: the main importer used for GitHub projects
* Gitlab::GithubImport: importer that's somewhat confusingly used for
  importing Gitea projects (apparently they have a compatible API)

This MR renames the Gitea importer to Gitlab::LegacyGithubImport and
introduces a new GitHub importer in the Gitlab::GithubImport namespace.
This new GitHub importer uses Sidekiq for importing multiple resources
in parallel, though it also has the ability to import data sequentially
should this be necessary.

The new code is spread across the following directories:

* lib/gitlab/github_import: this directory contains most of the importer
  code such as the classes used for importing resources.
* app/workers/gitlab/github_import: this directory contains the Sidekiq
  workers, most of which simply use the code from the directory above.
* app/workers/concerns/gitlab/github_import: this directory provides a
  few modules that are included in every GitHub importer worker.

== Stages

The import work is divided into separate stages, with each stage
importing a specific set of data. Stages will schedule the work that
needs to be performed, followed by scheduling a job for the
"AdvanceStageWorker" worker. This worker will periodically check if all
work is completed and schedule the next stage if this is the case. If
work is not yet completed this worker will reschedule itself.

Using this approach we don't have to block threads by calling `sleep()`,
as doing so for large projects could block the thread from doing any
work for many hours.

== Retrying Work

Workers will reschedule themselves whenever necessary. For example,
hitting the GitHub API's rate limit will result in jobs rescheduling
themselves. These jobs are not processed until the rate limit has been
reset.

== User Lookups

Part of the importing process involves looking up user details in the
GitHub API so we can map them to GitLab users. The old importer used
an in-memory cache, but this obviously doesn't work when the work is
spread across different threads.

The new importer uses a Redis cache and makes sure we only perform
API/database calls if absolutely necessary.  Frequently used keys are
refreshed, and lookup misses are also cached; removing the need for
performing API/database calls if we know we don't have the data we're
looking for.

== Performance & Models

The new importer in various places uses raw INSERT statements (as
generated by `Gitlab::Database.bulk_insert`) instead of using Rails
models. This allows us to bypass any validations and callbacks,
drastically reducing the number of SQL queries and Gitaly RPC calls
necessary to import projects.

To ensure the code produces valid data the corresponding tests check if
the produced rows are valid according to the model validation rules.

Signed-off-by: NRémy Coutable <remy@rymai.me>