From bc6806aa896177dbf919c03abc19d80b633cfbdc Mon Sep 17 00:00:00 2001
From: Georg Gadinger <nilsding@nilsding.org>
Date: Mon, 13 Feb 2023 20:13:32 +0100
Subject: [PATCH] initial metrics

---
 Gemfile                                     |  2 ++
 Gemfile.lock                                |  2 ++
 app/controllers/inbox_controller.rb         | 11 +++++++
 app/controllers/metrics_controller.rb       | 17 ++++++++++
 config/routes.rb                            |  3 ++
 lib/retrospring/metrics.rb                  | 36 +++++++++++++++++++++
 lib/use_case/question/create.rb             | 11 +++++++
 lib/use_case/question/create_followers.rb   | 11 +++++++
 spec/controllers/metrics_controller_spec.rb | 16 +++++++++
 9 files changed, 109 insertions(+)
 create mode 100644 app/controllers/metrics_controller.rb
 create mode 100644 lib/retrospring/metrics.rb
 create mode 100644 spec/controllers/metrics_controller_spec.rb

diff --git a/Gemfile b/Gemfile
index 1b14f588..fb0afd81 100644
--- a/Gemfile
+++ b/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"
diff --git a/Gemfile.lock b/Gemfile.lock
index 8fd5df6d..f7d3b596 100644
--- a/Gemfile.lock
+++ b/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)
diff --git a/app/controllers/inbox_controller.rb b/app/controllers/inbox_controller.rb
index d849dafc..16c47099 100644
--- a/app/controllers/inbox_controller.rb
+++ b/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
diff --git a/app/controllers/metrics_controller.rb b/app/controllers/metrics_controller.rb
new file mode 100644
index 00000000..dae673a6
--- /dev/null
+++ b/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
diff --git a/config/routes.rb b/config/routes.rb
index 744a8d83..fc210a32 100644
--- a/config/routes.rb
+++ b/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
diff --git a/lib/retrospring/metrics.rb b/lib/retrospring/metrics.rb
new file mode 100644
index 00000000..0072fd81
--- /dev/null
+++ b/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
diff --git a/lib/use_case/question/create.rb b/lib/use_case/question/create.rb
index 5cc8f174..a7c1345d 100644
--- a/lib/use_case/question/create.rb
+++ b/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?) ||
diff --git a/lib/use_case/question/create_followers.rb b/lib/use_case/question/create_followers.rb
index 21c2e89d..a6bddb0f 100644
--- a/lib/use_case/question/create_followers.rb
+++ b/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
diff --git a/spec/controllers/metrics_controller_spec.rb b/spec/controllers/metrics_controller_spec.rb
new file mode 100644
index 00000000..c278e308
--- /dev/null
+++ b/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