feat: add optional Prometheus metrics collection

This collector can be used by apps using yabeda and yabeda-rails gems.

For example, in initializers/apia.rb

require 'apia/prometheus_collector'

lib/apia/prometheus_collector.rb

@@ -0,0 +1,72 @@
+# frozen_string_literal: true
+
+unless defined?(Yabeda)
+  raise 'Yabeda is not defined. Please add the `yabeda` and `yabeda-rails` gems to your Gemfile.'
+end
+
+module Apia
+  class PrometheusCollector
+
+    def register_metrics
+      Yabeda.configure do
+        group :apia do
+          counter :requests_total do
+            comment 'The total number of requests handled by Apia'
+            tags([:method, :route, :status, :api, :endpoint, :authenticator, :error_code])
+          end
+          histogram :endpoint_duration do
+            comment 'Time spent in Apia requests in seconds'
+            unit :seconds
+            tags([:status, :endpoint])
+            buckets Yabeda::Rails::LONG_RUNNING_REQUEST_BUCKETS
+          end
+        end
+      end
+    end
+
+    def start
+      register_metrics
+      ActiveSupport::Notifications.subscribe(/(request(_error)?)\.apia/) do |*args|
+        event = ActiveSupport::Notifications::Event.new(*args)
+
+        increment_request_total_metric(event)
+        observe_endpoint_duration_metric(event)
+      end
+    end
+
+    private
+
+    def increment_request_total_metric(event)
+      tags = {
+        endpoint: event.payload[:request]&.endpoint&.name,
+        status: event.payload[:response]&.status,
+        api: event.payload[:request]&.api&.name,
+        route: "/#{event.payload[:request]&.route&.path}",
+        method: event.payload[:request]&.route&.request_method,
+        authenticator: event.payload[:request]&.authenticator&.name,
+        error_code: extract_error_code_from_response_body(event.payload[:response]&.body)
+      }
+      Yabeda.apia.requests_total.increment(tags, by: 1)
+    end
+
+    def observe_endpoint_duration_metric(event)
+      return if event.payload[:time].nil?
+
+      tags = {
+        endpoint: event.payload[:request]&.endpoint&.name,
+        status: event.payload[:response]&.status
+      }
+      Yabeda.apia.endpoint_duration.measure(tags, event.payload[:time])
+    end
+
+    def extract_error_code_from_response_body(body)
+      return '' unless body.is_a?(Hash)
+
+      body.dig(:error, :code).to_s
+    end
+
+  end
+end
+
+collector = Apia::PrometheusCollector.new
+collector.start