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`.

4 files changed, 45 insertions, 0 deletions

diff --git a/app/controllers/sherlock/application_controller.rb b/app/controllers/sherlock/application_controller.rb
new file mode 100644
index 00000000000..682ca5e3821
--- /dev/null
+++ b/app/controllers/sherlock/application_controller.rb
@@ -0,0 +1,12 @@
+module Sherlock
+  class ApplicationController < ::ApplicationController
+    before_action :find_transaction
+
+    def find_transaction
+      if params[:transaction_id]
+        @transaction = Gitlab::Sherlock.collection.
+          find_transaction(params[:transaction_id])
+      end
+    end
+  end
+end
diff --git a/app/controllers/sherlock/file_samples_controller.rb b/app/controllers/sherlock/file_samples_controller.rb
new file mode 100644
index 00000000000..0c3bc100106
--- /dev/null
+++ b/app/controllers/sherlock/file_samples_controller.rb
@@ -0,0 +1,7 @@
+module Sherlock
+  class FileSamplesController < Sherlock::ApplicationController
+    def show
+      @file_sample = @transaction.find_file_sample(params[:id])
+    end
+  end
+end
diff --git a/app/controllers/sherlock/queries_controller.rb b/app/controllers/sherlock/queries_controller.rb
new file mode 100644
index 00000000000..63b26aab1a4
--- /dev/null
+++ b/app/controllers/sherlock/queries_controller.rb
@@ -0,0 +1,7 @@
+module Sherlock
+  class QueriesController < Sherlock::ApplicationController
+    def show
+      @query = @transaction.find_query(params[:id])
+    end
+  end
+end
diff --git a/app/controllers/sherlock/transactions_controller.rb b/app/controllers/sherlock/transactions_controller.rb
new file mode 100644
index 00000000000..ccc739da879
--- /dev/null
+++ b/app/controllers/sherlock/transactions_controller.rb
@@ -0,0 +1,19 @@
+module Sherlock
+  class TransactionsController < Sherlock::ApplicationController
+    def index
+      @transactions = Gitlab::Sherlock.collection.newest_first
+    end
+
+    def show
+      @transaction = Gitlab::Sherlock.collection.find_transaction(params[:id])
+
+      render_404 unless @transaction
+    end
+
+    def destroy_all
+      Gitlab::Sherlock.collection.clear
+
+      redirect_to(:back)
+    end
+  end
+end