The use of ActiveSupport would slow down instrumented method calls by
about 180x due to:

1. ActiveSupport itself not being the fastest thing on the planet
2. caller_locations() having quite some overhead

The use of caller_locations() has been removed because it's not _that_
useful since we already know the full namespace of receivers and the
names of the called methods.

The use of ActiveSupport has been replaced with some custom code that's
generated using eval() (which can be quite a bit faster than using
define_method).

This new setup results in instrumented methods only being about 35-40x
slower (compared to non instrumented methods).

This is faster than using define_method since we don't have to keep
block bindings around.

The previous setup wasn't exactly fast, resulting in instrumented method
calls taking about 600 times longer than non instrumented calls
(including any ActiveSupport code involved). With this commit this
slowdown has been reduced to around 185 times.

This adds the ability to write application metrics (e.g. SQL timings) to
InfluxDB. These metrics can in turn be visualized using Grafana, or
really anything else that can read from InfluxDB. These metrics can be
used to track application performance over time, between different Ruby
versions, different GitLab versions, etc.

== Transaction Metrics

Currently the following is tracked on a per transaction basis (a
transaction is a Rails request or a single Sidekiq job):

* Timings per query along with the raw (obfuscated) SQL and information
  about what file the query originated from.
* Timings per view along with the path of the view and information about
  what file triggered the rendering process.
* The duration of a request itself along with the controller/worker
  class and method name.
* The duration of any instrumented method calls (more below).

== Sampled Metrics

Certain metrics can't be directly associated with a transaction. For
example, a process' total memory usage is unrelated to any running
transactions. While a transaction can result in the memory usage going
up there's no accurate way to determine what transaction is to blame,
this becomes especially problematic in multi-threaded environments.

To solve this problem there's a separate thread that takes samples at a
fixed interval. This thread (using the class Gitlab::Metrics::Sampler)
currently tracks the following:

* The process' total memory usage.
* The number of file descriptors opened by the process.
* The amount of Ruby objects (using ObjectSpace.count_objects).
* GC statistics such as timings, heap slots, etc.

The default/current interval is 15 seconds, any smaller interval might
put too much pressure on InfluxDB (especially when running dozens of
processes).

== Method Instrumentation

While currently not yet used methods can be instrumented to track how
long they take to run. Unlike the likes of New Relic this doesn't
require modifying the source code (e.g. including modules), it all
happens from the outside. For example, to track `User.by_login` we'd add
the following code somewhere in an initializer:

    Gitlab::Metrics::Instrumentation.
      instrument_method(User, :by_login)

to instead instrument an instance method:

    Gitlab::Metrics::Instrumentation.
      instrument_instance_method(User, :save)

Instrumentation for either all public model methods or a few crucial
ones will be added in the near future, I simply haven't gotten to doing
so just yet.

== Configuration

By default metrics are disabled. This means users don't have to bother
setting anything up if they don't want to. Metrics can be enabled by
editing one's gitlab.yml configuration file (see
config/gitlab.yml.example for example settings).

== Writing Data To InfluxDB

Because InfluxDB is still a fairly young product I expect the worse.
Data loss, unexpected reboots, the database not responding, you name it.
Because of this data is _not_ written to InfluxDB directly, instead it's
queued and processed by Sidekiq. This ensures that users won't notice
anything when InfluxDB is giving trouble.

The metrics worker can be started in a standalone manner as following:

    bundle exec sidekiq -q metrics

The corresponding class is called MetricsWorker.

Use gitlab-workhorse 0.5.1

Improves relative URL support.

See merge request !2134

Upgrade Poltergeist to 1.8.1.

Close #4131 

See merge request !2126

Fix ci_projects migration by using the value only from latest row [ci skip]

This is needed, because for some projects we have duplicate ci_projects.
This was introduced by lack of DB uniqueness on ci_projects.gitlab_id.

The MR fixes edge case by selecting value from latest.

/cc @yorickpeterse 

Fixes: gitlab-org/gitlab-ce#4123


See merge request !2133

Revert sidebar position for issue and merge request
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

cc @creamzy @skyruler 

See merge request !2132

This is needed, because for some projects we have duplicate ci_projects.
This was introduced by lack of DB uniqueness on ci_projects.gitlab_id.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Fix ux issue with "This issue will be closed automatically" message

1. Make a nice box for "issue will be closed automatically" message
2. move it below emoji navbar. Otherwise looks wierd. 

Fixes #4134 

# Before
![Screenshot_2015-12-17_00.01.03](/uploads/2a24de77f76ce8a8b61ebabd3a02c83a/Screenshot_2015-12-17_00.01.03.png)

# After

![Screenshot_2015-12-17_00.03.08](/uploads/ccd21b87b7509e79773fabd179bef4aa/Screenshot_2015-12-17_00.03.08.png)
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !2129

Add info on using private Docker registries in CI



See merge request !2130

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Api support for requesting starred projects for user

Closes #4112

Note: probably targets the wrong release in the `CHANGELOG`, though 8.4 was not there yet

See merge request !2127

Move MR Builds tab next to Commits

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/4132

@dzaporozhets We've got two people who feel strongly about this now, @sytses filed an issue for it :)

![Screen_Shot_2015-12-16_at_22.19.35](/uploads/37e705610ba54e386928d7972398c247/Screen_Shot_2015-12-16_at_22.19.35.png)


See merge request !2128

Fix updating and disabling merge button when Merge Immediately is clicked

Fixes #4128

See merge request !2124

Fix Rubocop complain.

See merge request !2125

Fixes #4112

Clean up ci yaml doc

* Fix headings so that they are picked up by doc.gitlab.com
* Stick to 80 characters
* Clean up some examples

See merge request !2052

Update rerun to remove celluloid as dependency

After sidekiq 4 migration, we no longer need celluloid. `rerun` recent version also removed it from its dependency, so by getting this MR merged, we can solve #3797 

See merge request !2088

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Link to correct (source) project on MR builds tab

Used to link to `gitlab-org/gitlab-ce/commits/<branch-name>`, now links to correct project.

See merge request !2115

Temporarily disable Markdown caching



See merge request !2120

Fix headers of milestone and snippet show pages

Fix layout, and move "Edit" button to very right just like issue/MR.

Before:

![Screen_Shot_2015-12-15_at_17.33.20](/uploads/dbbeb6868b80e21894159126a0feca6a/Screen_Shot_2015-12-15_at_17.33.20.png)

![Screen_Shot_2015-12-15_at_17.33.28](/uploads/53d10b42adc34e48b063889120e9a746/Screen_Shot_2015-12-15_at_17.33.28.png)

After:

![Screen_Shot_2015-12-15_at_17.34.25](/uploads/b0d0ea3f6a7b61d51b474419207f7a02/Screen_Shot_2015-12-15_at_17.34.25.png)

![Screen_Shot_2015-12-15_at_17.35.55](/uploads/65b922f5249655d3733740690577c9d7/Screen_Shot_2015-12-15_at_17.35.55.png)

cc @skyruler 

cc @rspeicher 8.3 please!

See merge request !2104

Only cache markdown when object has been saved and has a proper cache_key.

See merge request !2119

Fix 'Merge Immediately' button

Fixes #4125

See merge request !2117

* Fix headings so that they are picked up by doc.gitlab.com
* Stick to 80 characters
* Clean up some examples

Fix bad merge

/cc: @dzaporozhets 

See merge request !2116

Fix endpoint not setting needed @ci_commit

/cc @DouweM

See merge request !2112