initial metrics

Gemfile

@@ -117,3 +117,5 @@ gem "openssl", "~> 3.1"
 
 # mail 2.8.0 breaks sendmail usage: https://github.com/mikel/mail/issues/1538
 gem "mail", "~> 2.7.1"
+
+gem "prometheus-client", "~> 4.0"

Gemfile.lock

@@ -279,6 +279,7 @@ GEM
     pg (1.4.5)
     pghero (3.1.0)
       activerecord (>= 6)
+    prometheus-client (4.0.0)
     public_suffix (4.0.7)
     puma (6.1.0)
       nio4r (~> 2.0)
@@ -522,6 +523,7 @@ DEPENDENCIES
   openssl (~> 3.1)
   pg
   pghero
+  prometheus-client (~> 4.0)
   puma
   pundit (~> 2.3)
   questiongenerator (~> 1.1)

app/controllers/inbox_controller.rb

@@ -37,6 +37,7 @@ class InboxController < ApplicationController
                                 user:                current_user)
 
     inbox = Inbox.create!(user: current_user, question_id: question.id, new: true)
+    increment_metric
 
     respond_to do |format|
       format.turbo_stream do
@@ -85,4 +86,14 @@ class InboxController < ApplicationController
     # using .dup to not modify @inbox -- useful in tests
     @inbox&.dup&.update_all(new: false) # rubocop:disable Rails/SkipsModelValidations
   end
+
+  def increment_metric
+    Retrospring::Metrics::QUESTIONS_ASKED.increment(
+      labels: {
+        anonymous: true,
+        followers: false,
+        generated: true,
+      }
+    )
+  end
 end

app/controllers/metrics_controller.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+require "prometheus/client/formats/text"
+
+class MetricsController < ActionController::API
+  include ActionController::MimeResponds
+
+  def show
+    render plain: metrics
+  end
+
+  private
+
+  def metrics
+    Prometheus::Client::Formats::Text.marshal(Prometheus::Client.registry)
+  end
+end

config/routes.rb

@@ -49,6 +49,9 @@ Rails.application.routes.draw do
   get "/linkfilter", to: "link_filter#index", as: :linkfilter
   get "/manifest.json", to: "manifests#show", as: :webapp_manifest
 
+  # TODO: limit this endpoint
+  get "/metrics", to: "metrics#show"
+
   # Devise routes
   devise_for :users, path: "user", skip: %i[sessions registrations]
   as :user do

lib/retrospring/metrics.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+module Retrospring
+  module Metrics
+    PROMETHEUS = Prometheus::Client.registry
+
+    # avoid re-registering metrics to make autoreloader happy:
+    class << self
+      %i[counter gauge histogram summary].each do |meth|
+        define_method meth do |name, *args, **kwargs|
+          PROMETHEUS.public_send(meth, name, *args, **kwargs)
+        rescue Prometheus::Client::Registry::AlreadyRegisteredError
+          raise unless Rails.env.development?
+
+          PROMETHEUS.unregister name
+          retry
+        end
+      end
+    end
+
+    VERSION_INFO = gauge(
+      :retrospring_version_info,
+      docstring:     "Information about the currently running version",
+      labels:        [:version],
+      preset_labels: {
+        version: Retrospring::Version.to_s,
+      }
+    ).tap { _1.set 1 }
+
+    QUESTIONS_ASKED = counter(
+      :retrospring_questions_asked_total,
+      docstring: "How many questions got asked",
+      labels:    %i[anonymous followers generated]
+    )
+  end
+end

lib/use_case/question/create.rb

@@ -18,6 +18,7 @@ module UseCase
         return if filtered?(question)
 
         increment_asked_count
+        increment_metric
 
         inbox = ::Inbox.create!(user: target_user, question:, new: true)
         notify(inbox)
@@ -92,6 +93,16 @@ module UseCase
         source_user.save
       end
 
+      def increment_metric
+        Retrospring::Metrics::QUESTIONS_ASKED.increment(
+          labels: {
+            anonymous:,
+            followers: false,
+            generated: false,
+          }
+        )
+      end
+
       def filtered?(question)
         target_user.mute_rules.any? { |rule| rule.applies_to? question } ||
           (anonymous && AnonymousBlock.where(identifier: question.author_identifier, user_id: [target_user.id, nil]).any?) ||

lib/use_case/question/create_followers.rb

@@ -17,6 +17,7 @@ module UseCase
         )
 
         increment_asked_count
+        increment_metric
 
         QuestionWorker.perform_async(source_user_id, question.id)
 
@@ -33,6 +34,16 @@ module UseCase
         source_user.save
       end
 
+      def increment_metric
+        Retrospring::Metrics::QUESTIONS_ASKED.increment(
+          labels: {
+            anonymous: false,
+            followers: true,
+            generated: false,
+          }
+        )
+      end
+
       def source_user
         @source_user ||= ::User.find(source_user_id)
       end

spec/controllers/metrics_controller_spec.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+describe MetricsController, type: :controller do
+  describe "#show" do
+    subject { get :show }
+
+    it "returns the metrics" do
+      # ensure we have at least a metric set
+      Retrospring::Metrics::VERSION_INFO.set 1
+
+      expect(subject.body).to include "retrospring_version_info"
+    end
+  end
+end