puma: add Prometheus metrics exporter.

To test it, run `bin/rails server` then do `curl http://localhost:9393`.

https://github.com/harmjanblok/puma-metrics

Gemfile

@@ -53,6 +53,7 @@ gem 'bcrypt_pbkdf' # https://github.com/net-ssh/net-ssh/issues/565
 gem 'terminal-table'
 gem 'newrelic_rpm', require: false
 gem 'clockwork'
+gem 'puma-metrics'
 
 group :production, :staging do
   gem 'unicorn', :platforms => :ruby

Gemfile.lock

@@ -316,6 +316,7 @@ GEM
     parser (3.0.2.0)
       ast (~> 2.4.1)
     pg (1.2.3)
+    prometheus-client (2.1.0)
     pry (0.13.1)
       coderay (~> 1.1)
       method_source (~> 1.0)
@@ -327,6 +328,9 @@ GEM
     public_suffix (4.0.6)
     puma (5.4.0)
       nio4r (~> 2.0)
+    puma-metrics (1.2.3)
+      prometheus-client (>= 0.10)
+      puma (>= 3.0)
     pundit (2.1.1)
       activesupport (>= 3.0.0)
     racc (1.5.2)
@@ -557,6 +561,7 @@ DEPENDENCIES
   pry-byebug
   pry-rails
   puma
+  puma-metrics
   pundit
   rack-mini-profiler
   rails (~> 6.0)

config/puma.rb

@@ -12,6 +12,7 @@
 # * PUMA_WORKER_TIMEOUT
 # * PUMA_PIDFILE
 # * PUMA_CONTROL_URL
+# * PUMA_METRICS_URL
 #
 # Use `bin/pumactl` to control a running Puma instance.
 #
@@ -84,6 +85,13 @@ end
 # Allow puma to be restarted by `rails restart` command.
 plugin :tmp_restart
 
+# Enable Prometheus metrics.
+# https://github.com/harmjanblok/puma-metrics
+plugin :metrics
+
+# Export Prometheus metrics by default on http://localhost:9393
+metrics_url ENV.fetch("PUMA_METRICS_URL", "tcp://localhost:9393")
+
 # Start the Puma control rack application on +url+. This application can
 # be communicated with to control the main server. Additionally, you can
 # provide an authentication token, so all requests to the control server