puma: add Prometheus metrics exporter.

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

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

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