Instrument EmailDigestArticleCollector (#13153)

* Instrument EmailDigestArticleCollector

On DEV, since we skip this with sidekiq-cron and instead process them
inline using Heroku Scheduler (which is not instrumented), we miss out
on all instrumentation for this process and have no idea how long it
takes to process each user.

With this instrumentation in place, we can see how long it takes to
process each user and aggregate them to see how long it takes to process
all users.

This commit also adds an Instrumentation mixin that we can use to
instrument blocks easily:

    instrument "MyClass.my_method", tags: { article: article.id } do |span|
      # ...
    end

* Add Instrumentation mixin

Oops, completely forgot to commit this file

app/services/email_digest_article_collector.rb

@@ -1,39 +1,47 @@
 class EmailDigestArticleCollector
+  include Instrumentation
+
+  ARTICLES_TO_SEND = "EmailDigestArticleCollector#articles_to_send".freeze
+
   def initialize(user)
     @user = user
   end
 
   def articles_to_send
-    return [] unless should_receive_email?
+    # rubocop:disable Metrics/BlockLength
+    instrument ARTICLES_TO_SEND, tags: %W[user_id:#{@user.id}] do
+      return [] unless should_receive_email?
 
-    articles = if user_has_followings?
-                 experience_level_rating = (@user.experience_level || 5)
-                 experience_level_rating_min = experience_level_rating - 3.6
-                 experience_level_rating_max = experience_level_rating + 3.6
+      articles = if user_has_followings?
+                   experience_level_rating = (@user.experience_level || 5)
+                   experience_level_rating_min = experience_level_rating - 3.6
+                   experience_level_rating_max = experience_level_rating + 3.6
 
-                 @user.followed_articles
-                   .select(:title, :description, :path)
-                   .published
-                   .where("published_at > ?", cutoff_date)
-                   .where(email_digest_eligible: true)
-                   .where.not(user_id: @user.id)
-                   .where("score > ?", 12)
-                   .where("experience_level_rating > ? AND experience_level_rating < ?",
-                          experience_level_rating_min, experience_level_rating_max)
-                   .order(score: :desc)
-                   .limit(6)
-               else
-                 Article.select(:title, :description, :path)
-                   .published
-                   .where("published_at > ?", cutoff_date)
-                   .where(featured: true, email_digest_eligible: true)
-                   .where.not(user_id: @user.id)
-                   .where("score > ?", 25)
-                   .order(score: :desc)
-                   .limit(6)
-               end
+                   @user.followed_articles
+                     .select(:title, :description, :path)
+                     .published
+                     .where("published_at > ?", cutoff_date)
+                     .where(email_digest_eligible: true)
+                     .where.not(user_id: @user.id)
+                     .where("score > ?", 12)
+                     .where("experience_level_rating > ? AND experience_level_rating < ?",
+                            experience_level_rating_min, experience_level_rating_max)
+                     .order(score: :desc)
+                     .limit(6)
+                 else
+                   Article.select(:title, :description, :path)
+                     .published
+                     .where("published_at > ?", cutoff_date)
+                     .where(featured: true, email_digest_eligible: true)
+                     .where.not(user_id: @user.id)
+                     .where("score > ?", 25)
+                     .order(score: :desc)
+                     .limit(6)
+                 end
 
-    articles.length < 3 ? [] : articles
+      articles.length < 3 ? [] : articles
+    end
+    # rubocop:enable Metrics/BlockLength
   end
 
   private
@@ -81,7 +89,10 @@ class EmailDigestArticleCollector
   end
 
   def last_user_emails
-    @last_user_emails ||= @user.email_messages.select(:sent_at,
-                                                      :opened_at).where(mailer: "DigestMailer#digest_email").limit(10)
+    @last_user_emails ||= @user
+      .email_messages
+      .select(:sent_at, :opened_at)
+      .where(mailer: "DigestMailer#digest_email")
+      .limit(10)
   end
 end

app/services/instrumentation.rb

@@ -0,0 +1,7 @@
+module Instrumentation
+  def instrument(operation, tags: [], &block)
+    # TODO: (@jgaskins): Extract the knowledge of which tracing library
+    # we're using, like we did with ForemStatsDriver
+    Datadog.tracer.trace operation, tags: tags, &block
+  end
+end