Adds barnes gem to collect Ruby Language Metrics

Why are these changes being introduced:

* This adds a few additional useful metrics in our heroku dashboard
  for stage/prod level apps (it doesn't work in eco dynos so pr
  builds will not collect this data)
* see also:
  https://devcenter.heroku.com/articles/language-runtime-metrics-ruby

Relevant ticket(s):

* https://mitlibraries.atlassian.net/browse/TCO-78

How does this address that need:

* adds barnes gem
* configures puma per heroku docs to enable barnes
* updates app.json to add the metrics collection to PR builds even
  though they probably won't do anything in our eco dynos

Document any side effects to this change:

None really, but the only other rails app we do this with is Bento.
Other apps should likely get updated to collect this data if we don't
see any side effects.

Gemfile

@@ -6,6 +6,9 @@ git_source(:github) do |repo_name|
   "https://github.com/#{repo_name}.git"
 end
 
+# GC Statsd Reporter [https://github.com/heroku/barnes]
+gem 'barnes'
+
 # Reduces boot times through caching; required in config/boot.rb
 gem 'bootsnap', require: false
 

Gemfile.lock

@@ -100,6 +100,9 @@ GEM
       rake (>= 10.4, < 14.0)
     ast (2.4.2)
     attr_required (1.0.2)
+    barnes (0.0.9)
+      multi_json (~> 1)
+      statsd-ruby (~> 1.1)
     base64 (0.2.0)
     bcrypt (3.1.20)
     bigdecimal (3.1.8)
@@ -230,6 +233,7 @@ GEM
     mini_mime (1.1.5)
     minitest (5.25.1)
     msgpack (1.7.2)
+    multi_json (1.15.0)
     mutex_m (0.2.0)
     net-http (0.4.1)
       uri
@@ -409,6 +413,7 @@ GEM
     sqlite3 (1.7.3-arm64-darwin)
     sqlite3 (1.7.3-x86_64-darwin)
     sqlite3 (1.7.3-x86_64-linux)
+    statsd-ruby (1.5.0)
     stimulus-rails (1.3.4)
       railties (>= 6.0.0)
     stringex (2.8.6)
@@ -469,6 +474,7 @@ PLATFORMS
 DEPENDENCIES
   administrate (~> 0.20.1)
   annotate
+  barnes
   bootsnap
   cancancan
   capybara

app.json

@@ -26,6 +26,9 @@
   "buildpacks": [
     {
       "url": "heroku/ruby"
+    },
+    {
+      "url": "heroku/metrics"
     }
   ]
 }

config/puma.rb

@@ -33,3 +33,11 @@
 
 # Allow puma to be restarted by `bin/rails restart` command.
 plugin :tmp_restart
+
+require 'barnes'
+
+before_fork do
+  # worker specific setup
+
+  Barnes.start # Must have enabled worker mode for this to block to be called
+end