Added Sherlock, a custom profiling tool for GitLab

Sherlock will be a new GitLab specific tool for measuring the
performance of Rails requests (and SideKiq jobs at some point). Some of
the things that are currently tracked:

* SQL queries along with their timings, backtraces and query plans
  (using "EXPLAIN ANALYZE" for PostgreSQL and regular "EXPLAIN" for
  MySQL)
* Timings of application files (including views) on a per line basis
* Some meta data such as the request method, path, total duration, etc

More tracking (e.g. Rugged or gitlab-shell timings) might be added in
the future.

Sherlock will replace any existing tools we have used so far (e.g.
active_record_query_trace and rack-mini-profiler), hence the
corresponding Gems have been removed from the Gemfile.

Sherlock can be enabled by starting Rails as following:

    ENABLE_SHERLOCK=1 bundle exec rails s

Recorded transactions can be found at `/sherlock/transactions`.

1 files changed, 5 insertions, 0 deletions

diff --git a/config/initializers/sherlock.rb b/config/initializers/sherlock.rb
new file mode 100644
index 00000000000..42b0d78c85f
--- /dev/null
+++ b/config/initializers/sherlock.rb
@@ -0,0 +1,5 @@
+if Gitlab::Sherlock.enabled?
+  Gitlab::Application.configure do |config|
+    config.middleware.use(Gitlab::Sherlock::Middleware)
+  end
+end