Preload the app in `production` env, hit it with a single request, and
gather total gem memory consumption data using `derailed exec perf:mem`
from `derailed_benchmarks`. Present the result as MR metrics.

This reverts merge request !30031

Preload the app in `production` env, hit it with a single request, and
gather total gem memory consumption data using `derailed exec perf:mem`
from `derailed_benchmarks`. Present the result as MR metrics.

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

* updates Gemfile
* uses Rails 5 unless explicitly disabled

This will enable bootsnap whenver the gem is available.

This reverts commit 83ec509c, reversing
changes made to 57f0677a.

This reverts commit 95138a9c, reversing
changes made to 66aad16d.

+ Set defaults correctly only for when not in production or staging
+ set ENV['prometheus_multiproc_dir'] in config/boot.rb instead of config.ru

Test prometheus metrics unmemoized

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

This reports uses of `File.exists?` and `Dir.exists?`, which were both
deprecated in Ruby and will eventually be removed in favor of
`.exist?`. Also fixes all existing uses of the deprecated methods.