Fix missing gem requires

We need to manually require stdlib classes and classes from gems
that don't get required by default in the Gemfile

Gemfile

@@ -475,7 +475,7 @@ gem 'lograge', '~> 0.5'
 gem 'grape_logging', '~> 1.7'
 
 # DNS Lookup
-gem 'gitlab-net-dns', '~> 0.9.1'
+gem 'gitlab-net-dns', '~> 0.9.1', require: 'net/dns'
 
 # Countries list
 gem 'countries', '~> 3.0'

lib/gitlab/metrics/exporter/base_exporter.rb

 # frozen_string_literal: true
 
+require 'webrick'
+require 'prometheus/client/rack/exporter'
+
 module Gitlab
   module Metrics
     module Exporter

spec/lib/gitlab/fogbugz_import/importer_spec.rb

 # frozen_string_literal: true
 
 require 'spec_helper'
+require 'fogbugz'
 
 describe Gitlab::FogbugzImport::Importer do
   let(:project) { create(:project_empty_repo) }