When prometheus is not installed, Rails won't boot:

 such file to load -- prometheus/client.rb (LoadError)
  /usr/share/foreman/vendor/ruby/2.3.0/gems/polyglot-0.3.5/lib/polyglot.rb:65:in `require'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/polyglot-0.3.5/lib/polyglot.rb:65:in `require'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `block in require'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:258:in `load_dependency'
  /usr/share/foreman/vendor/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `require'
  /usr/share/foreman/lib/foreman/telemetry_sinks/prometheus_sink.rb:1:in `<top (required)>&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/polyglot-0.3.5/lib/polyglot.rb:65:in `require&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/polyglot-0.3.5/lib/polyglot.rb:65:in `require&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `block in require&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:258:in `load_dependency&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:292:in `require&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:379:in `block in require_or_load&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:36:in `block in load_interlock&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/dependencies/interlock.rb:12:in `block in loading&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/concurrency/share_lock.rb:149:in `exclusive&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/dependencies/interlock.rb:11:in `loading&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:36:in `load_interlock&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:357:in `require_or_load&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:335:in `depend_on&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/activesupport-5.1.4/lib/active_support/dependencies.rb:251:in `require_dependency&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/engine.rb:476:in `block (2 levels) in eager_load!&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/engine.rb:475:in `each&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/engine.rb:475:in `block in eager_load!&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/engine.rb:473:in `each&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/engine.rb:473:in `eager_load!&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/engine.rb:354:in `eager_load!&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/application/finisher.rb:67:in `each&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/application/finisher.rb:67:in `block in <module:Finisher>&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/initializable.rb:30:in `instance_exec&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/initializable.rb:30:in `run&apos;
  /usr/share/foreman/vendor/ruby/2.3.0/gems/railties-5.1.4/lib/rails/initializable.rb:59:in `block in run_initializers&apos;
  /usr/lib/ruby/2.3.0/tsort.rb:228:in `block in tsort_each&apos;
  /usr/lib/ruby/2.3.0/tsort.rb:350:in `block (2 levels) in each_strongly_connected_com