Instrument all ActiveRecord model methods

This works by searching the raw source code for any references to
commonly used ActiveRecord methods. While not bulletproof it saves us
from having to list hundreds of methods by hand. It also ensures that
(most) newly added methods are instrumented automatically.

This _only_ instruments models defined in app/models, should a model
reside somewhere else (e.g. somewhere in lib/) it _won't_ be
instrumented.

Gemfile

@@ -210,6 +210,7 @@ gem 'net-ssh',            '~> 3.0.1'
 # Metrics
 group :metrics do
+  gem 'method_source', '~> 0.8', require: false
   gem 'influxdb', '~> 0.2', require: false
   gem 'connection_pool', '~> 2.0', require: false
 end

Gemfile.lock

@@ -887,6 +887,7 @@ DEPENDENCIES
   kaminari (~> 0.16.3)
   letter_opener (~> 1.1.2)
   mail_room (~> 0.6.1)
+  method_source (~> 0.8)
   minitest (~> 5.7.0)
   mousetrap-rails (~> 1.4.6)
   mysql2 (~> 0.3.16)

config/initializers/metrics.rb

@@ -2,6 +2,7 @@ if Gitlab::Metrics.enabled?
   require 'influxdb'
   require 'socket'
   require 'connection_pool'
+  require 'method_source'
   # These are manually require'd so the classes are registered properly with
   # ActiveSupport.
@@ -18,6 +19,26 @@ if Gitlab::Metrics.enabled?
     end
   end
+  # This instruments all methods residing in app/models that (appear to) use any
+  # of the ActiveRecord methods. This has to take place _after_ initializing as
+  # for some unknown reason calling eager_load! earlier breaks Devise.
+  Gitlab::Application.config.after_initialize do
+    Rails.application.eager_load!
+
+    models = Rails.root.join('app', 'models').to_s
+
+    regex = Regexp.union(
+      ActiveRecord::Querying.public_instance_methods(false).map(&:to_s)
+    )
+
+    Gitlab::Metrics::Instrumentation.
+      instrument_class_hierarchy(ActiveRecord::Base) do |_, method|
+        loc = method.source_location
+
+        loc && loc[0].start_with?(models) && method.source =~ regex
+      end
+  end
+
   Gitlab::Metrics::Instrumentation.configure do |config|
     config.instrument_instance_methods(Gitlab::Shell)