Show pg_stat_activity before each test run

Maybe this will help us debug
https://gitlab.com/gitlab-org/gitlab-ee/issues/10154

spec/support/pg_stat_activity.rb

+# frozen_string_literal: true
+
+RSpec.configure do |config|
+  config.before do
+    if Gitlab::Database.postgresql? && ENV['PG_STAT_WARNING_THRESHOLD']
+      warning_threshold = ENV['PG_STAT_WARNING_THRESHOLD'].to_i
+      results = ActiveRecord::Base.connection.execute('SELECT * FROM pg_stat_activity')
+      ntuples = results.ntuples
+
+      warn("pg_stat_activity count: #{ntuples}")
+
+      if ntuples > warning_threshold
+        results.each do |result|
+          warn result.inspect
+        end
+      end
+    end
+  end
+end