/usr/lib/ruby/vendor_ruby/ is in ruby-riemann-client 0.2.3-1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/vendor_ruby/riemann/
/usr/lib/ruby/vendor_ruby/riemann/metric_thread.rb
/usr/lib/ruby/vendor_ruby/riemann/event.rb
/usr/lib/ruby/vendor_ruby/riemann/auto_state.rb
/usr/lib/ruby/vendor_ruby/riemann/query.rb
/usr/lib/ruby/vendor_ruby/riemann/attribute.rb
/usr/lib/ruby/vendor_ruby/riemann/version.rb
/usr/lib/ruby/vendor_ruby/riemann/state.rb
/usr/lib/ruby/vendor_ruby/riemann/client/
/usr/lib/ruby/vendor_ruby/riemann/client/udp.rb
/usr/lib/ruby/vendor_ruby/riemann/client/tcp.rb
/usr/lib/ruby/vendor_ruby/riemann/client/tcp_socket.rb
/usr/lib/ruby/vendor_ruby/riemann/client.rb
/usr/lib/ruby/vendor_ruby/riemann/message.rb
/usr/lib/ruby/vendor_ruby/riemann.rb