Add barnes gem (needed to report ruby runtime metrics to heroku)

[eddierubeiz]

Ref #2449

Ref #2446

This PR adds the barnes gem, which reports Ruby runtime metrics to Heroku, where we can monitor them at e.g.

Note: to turn on ruby runtime metrics:

heroku labs:enable "runtime-heroku-metrics"
heroku buildpacks:add -i 1 heroku/metrics

After these are run and this PR is merged and deployed, you will see three more time series graphs on the Metrics tab for the app in Heroku's admin interface. (e.g. https://dashboard.heroku.com/apps/scihist-digicoll-staging/metrics/web), namely

• Ruby: Heap Objects
• Ruby: Puma Pool Usage
• Ruby: Free Memory Slots

https://devcenter.heroku.com/articles/language-runtime-metrics-ruby

[jrochkind]

Hmm. config/puma.rb isn’t actually the file we use on heroku! Please also add to config/heroku_puma.rb — I’m surprised we got metrics at all this way, wonder if maybe they aren’t reliable!

Perhaps we should add a big comment warning to config/puma.rb since we are doing something a bit odd here (maybe we shoudln't be and should just use config/puma.rb on heroku -- but I didn't want to run multiple puma workers in dev and test etc!)

[jrochkind]

thank you for url to heroku docs next to barnes before_fork line, love it!

[jrochkind]

The proper specific ticket ref for this PR was #2446, not the general RAM ticket. Fixed in description.