feat: add puma metrics gem

This allows to monitor PUMA via prometheus, providing insights for
health, scaling, etc.

Gemfile

@@ -64,4 +64,5 @@ end
 group :production do
   gem 'lograge', '~> 0.14.0'
   gem 'remote_syslog_logger'
+  gem 'puma-metrics', '~> 1.2.0'
 end

Gemfile.lock

@@ -312,11 +312,15 @@ GEM
       ast (~> 2.4.1)
       racc
     pg (1.4.5)
+    prometheus-client (4.2.2)
     psych (5.1.2)
       stringio
     public_suffix (5.0.3)
     puma (5.6.8)
       nio4r (~> 2.0)
+    puma-metrics (1.2.5)
+      prometheus-client (>= 0.10)
+      puma (>= 5.0)
     racc (1.7.3)
     rack (2.2.8.1)
     rack-oauth2 (2.2.0)
@@ -534,6 +538,7 @@ DEPENDENCIES
   pagy (~> 6.0, >= 6.0.0)
   pg
   puma (~> 5.6)
+  puma-metrics (~> 1.2.0)
   rails (~> 7.1.3)
   redis (~> 4.0)
   remote_syslog_logger

config/puma.rb

@@ -59,3 +59,10 @@
 
 # Allow puma to be restarted by `bin/rails restart` command.
 plugin :tmp_restart
+# Enable puma-metrics gem https://rubygems.org/gems/puma-metrics
+plugin 'metrics'
+# Bind the metric server to "url". "tcp://" is the only accepted protocol.
+#
+# The default is "tcp://0.0.0.0:9393", we bind it to localhost to be safe
+# by default.
+metrics_url 'tcp://127.0.0.1:9393'