Added documentation for GitLab Metrics

This adds documentation for the metrics code I recently added.

Two notes:

* I'm referring to this feature as "GitLab Metrics" everywhere simply
  because the code resides in the `Gitlab::Metrics` namespace. A
  different name is fine, though I think just "Metrics" might be a bit
  too abstract.
* Grafana isn't covered since it's fairly straight-forward to setup
  (there's nothing really special we can cover). I think we _should_
  cover it once we start shipping it with Omnibus.

I assigned @axil since he's our technical writer/documentation guru,
feel free to re-assign if needed.

cc @sytses @axil @dzaporozhets @JobV 

See merge request !2484

Fix cross projects detection when importing GitHub pull requests

Fixes #12456

See merge request !2523

[ci skip]

[ci skip]

Add syntax highlighting to diff view

Closes #3945 

See merge request !2109

Mention required Ruby version

Closes #12446

See merge request !2524

[ci skip]

Add note about minimum GitLab CI version

Documentation update: To use variables in `.gitlab-ci.yml` it's not enough to update the GitLab Runner, a minimum version of GitLab CI is needed as well.

See merge request !2514

Split from !2455

References #9092

See merge request !2515

Gracefully handle invalid UTF-8 sequences in Markdown links

Closes #6077

See merge request !2382

Add sentry integration

Sentry is an event logging platform primarily focused on capturing and
aggregating exceptions. With this MR it will be possible to log and
track exceptions from GitLab to Sentry.

https://gitlab.com/gitlab-com/operations/issues/39

See merge request !2485

Use ENV variables for Unicorn worker killer & increase default limits

Merge request that lead to this particular merge request:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2421

[ci skip]

See merge request !2513

Upgraded gitlab_git to fix issue #5858

This updates the dependency on gitlab_git and fixes #5858 - reproduced locally and issue disappears with the change.

Related MR that actually adds branch name support to commits_between: https://gitlab.com/gitlab-org/gitlab_git/merge_requests/61

See merge request !2516

Fixes tooltip doesn't duplicate the "me" name.

Checks that "me" is not in the array of names in tooltip

![emojilist](/uploads/20b1a39cda194358f9730ec3c6b86acb/emojilist.gif)

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/7978


See merge request !2454

Adds "t" as a shortcut for find file anywhere



See merge request !2507

Fixed invalid JSON in first JSON Example

Just a comma missing

See merge request !2487

Closes #6077

Update Issues/MRs updated_at field when notes are generated

Closes #3749 and #9130

See merge request !2493

Remove `Compressed to` column from build artifacts browser

See #10982.

See merge request !2509

Using this limit on GitLab.com it appears we're able to reduce response
timings by about 620 milliseconds compared to the previous limit.

See gitlab-org/gitlab-ce!2421 for more information.