add prometheus initialiser

this ensures it works great in multi-process setups (like with puma)

config/initializers/prometheus.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+return if Rails.env.test? # no need for the direct file store in testing
+
+require "prometheus/client/data_stores/direct_file_store"
+
+Rails.application.config.before_configuration do
+  dir = Rails.root.join("tmp/prometheus_metrics")
+  FileUtils.mkdir_p dir
+
+  Prometheus::Client.config.data_store = Prometheus::Client::DataStores::DirectFileStore.new(dir:)
+end
+
+Rails.application.config.after_initialize do
+  # ensure the version metric is populated
+  Retrospring::Metrics::VERSION_INFO
+end