Fix autoload not picking up prometheus.rb (#5478)

**Story card:**
[sc-13408](https://app.shortcut.com/simpledotorg/story/13408/migrate-pending-custom-metrics-to-prometheus-from-datadog)

## Because

The Zeitwork mode is failing to load the prometheus.rb file in
production environments. Even though other .rb files are working fine.

## This addresses

As a temporary fix, we'll move Prometheus within the Metrics module

lib/metrics.rb

@@ -48,4 +48,22 @@ def benchmark_and_record_metric(type, event, labels, description)
     elapsed_time_seconds = Process.clock_gettime(Process::CLOCK_MONOTONIC) - start
     record_metric(type, event, elapsed_time_seconds, labels, description)
   end
+
+  class Prometheus
+    include Singleton
+
+    def initialize
+      @client = PrometheusExporter::Client.default
+      @collectors = {}
+    end
+
+    def register(type, name, description = nil)
+      @collectors[name] ||= @client.register(type, name, description)
+      self
+    end
+
+    def observe(name, value, labels = {})
+      @collectors[name].observe(value, labels)
+    end
+  end
 end