Push fact check metrics to prometheus

Inspired by alphagov/search-api-v2#209

This adds the prometheus-client gem, and then uses it to send fact check emails metrics to prometheus via the pushgateway.

We can't use the conventional prometheus collector, because this code runs as a script, rather than a web process.

This should not be merged until we've set the PROMETHEUS_PUSHGATEWAY_URL environment variable.

Gemfile

@@ -31,6 +31,7 @@ gem "mousetrap-rails"
 gem "nested_form", git: "https://github.com/alphagov/nested_form.git", branch: "add-wrapper-class"
 gem "null_logger"
 gem "plek"
+gem "prometheus-client"
 gem "rails_autolink"
 gem "rest-client", require: false
 gem "sassc-rails"

Gemfile.lock

@@ -594,6 +594,8 @@ GEM
       ast (~> 2.4.1)
       racc
     plek (5.2.0)
+    prometheus-client (4.2.3)
+      base64
     prometheus_exporter (2.1.1)
       webrick
     pry (0.14.1)
@@ -850,6 +852,7 @@ DEPENDENCIES
   nested_form!
   null_logger
   plek
+  prometheus-client
   pry-byebug
   rack
   rails (= 7.0.8.3)

app/lib/fact_check_email_handler.rb

@@ -9,8 +9,9 @@
 class FactCheckEmailHandler
   attr_accessor :fact_check_config
 
-  def initialize(fact_check_config)
+  def initialize(fact_check_config, unprocessed_emails_gauge)
     @fact_check_config = fact_check_config
+    @unprocessed_emails_gauge = unprocessed_emails_gauge
   end
 
   def process_message(message)
@@ -35,7 +36,7 @@ def process
       unprocessed_emails_count += 1
     end
 
-    GovukStatsd.gauge("unprocessed_emails.count", unprocessed_emails_count)
+    @unprocessed_emails_gauge.set(unprocessed_emails_count)
   rescue StandardError => e
     # Occasionally, there is an error when connecting to the mailbox in production.
     # It seems a very transient error, and since the job is run every few minutes isn't really a problem, but if the

script/mail_fetcher

@@ -20,7 +20,12 @@ end
 
 Rails.logger.info "Running MailFetcher in #{Rails.env} mode - #{Time.zone.now.utc}"
 
-handler = FactCheckEmailHandler.new(Publisher::Application.fact_check_config)
+registry = Prometheus::Client.registry
+gauge = registry.gauge(
+  :publisher_fact_check_unprocessed_emails_total,
+  docstring: "Number of unprocessed fact check emails"
+)
+handler = FactCheckEmailHandler.new(Publisher::Application.fact_check_config, gauge)
 
 # The lock is created and belongs to this process for as long as the `life`.
 # When the block has finished executing, the lock is explicitly released.
@@ -50,3 +55,7 @@ rescue StandardError => e
 end
 
 Rails.logger.info "Finished running MailFetcher in #{Rails.env} mode - #{Time.zone.now.utc}"
+Prometheus::Client::Push.new(
+  job: "publisher_fact_check_emails",
+  gateway: ENV.fetch("PROMETHEUS_PUSHGATEWAY_URL"),
+).add(registry)

test/unit/lib/fact_check_email_handler_test.rb

@@ -1,29 +1,34 @@
 require "test_helper"
 
 class FactCheckEmailHandlerTest < ActiveSupport::TestCase
+  setup do
+    @gauge = stub()
+  end
+
   def handler
-    FactCheckEmailHandler.new(Publisher::Application.fact_check_config)
+    FactCheckEmailHandler.new(Publisher::Application.fact_check_config, @gauge)
   end
 
   test "#process ignores 'out of office' emails" do
     out_of_office_message = Mail.new { subject("Automatic reply: out of office") }
     Mail.stubs(:all).yields(out_of_office_message)
+    @gauge.expects(:set).with(0)
     handler.process
     assert out_of_office_message.is_marked_for_delete?
   end
 
-  test "#process sends count of unprocessed emails to Graphite" do
+  test "#process sends count of unprocessed emails to Prometheus" do
     processed_message = Mail.new { subject("Automatic reply: out of office") }
     unprocessed_message = Mail.new { subject("Any other subject") }
-    GovukStatsd.expects(:gauge).with("unprocessed_emails.count", 1)
+    @gauge.expects(:set).with(1)
     Mail.stubs(:all).multiple_yields(processed_message, unprocessed_message)
 
     handler.process
   end
 
-  test "#process does not send count of unprocessed emails to Graphite when emails cannot be retrieved" do
+  test "#process does not send count of unprocessed emails to Prometheus when emails cannot be retrieved" do
     Mail.stubs(:all).raises(StandardError)
-    GovukStatsd.expects(:gauge).never
+    @gauge.expects(:set).never
 
     handler.process
   end