Count the number of SQL queries per transaction

Fixes gitlab-org/gitlab-ce#15335

CHANGELOG

@@ -4,6 +4,7 @@ v 8.7.0 (unreleased)
   - Method instrumentation now uses Module#prepend instead of aliasing methods
   - Repository.clean_old_archives is now instrumented
   - Add support for environment variables on a job level in CI configuration file
+  - SQL query counts are now tracked per transaction
   - The Projects::HousekeepingService class has extra instrumentation
   - All service classes (those residing in app/services) are now instrumented
   - Developers can now add custom tags to transactions

lib/gitlab/metrics/subscribers/active_record.rb

@@ -9,6 +9,7 @@ module Gitlab
           return unless current_transaction
 
           current_transaction.increment(:sql_duration, event.duration)
+          current_transaction.increment(:sql_count, 1)
         end
 
         private

spec/lib/gitlab/metrics/subscribers/active_record_spec.rb

@@ -28,6 +28,9 @@ describe Gitlab::Metrics::Subscribers::ActiveRecord do
         expect(transaction).to receive(:increment).
           with(:sql_duration, 0.2)
 
+        expect(transaction).to receive(:increment).
+          with(:sql_count, 1)
+
         subscriber.sql(event)
       end
     end