Add missing require on start-up

This doesn't appear to be required when running standalone,
but was reported causing problems when running on Puma in
Omnibus:
https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/6090

bin/gitlab-exporter

@@ -3,6 +3,7 @@
 $LOAD_PATH.unshift File.expand_path("../lib", __dir__)
 
 require "rubygems"
+require "bundler"
 require "bundler/setup"
 require "optparse"
 require "gitlab_exporter"