Add prometheus collector for oldest OS Places postcode

lib/collectors/global_prometheus_collector.rb

@@ -1,37 +1,28 @@
 require "prometheus_exporter"
 require "prometheus_exporter/server"
 
-# Load Rails models as not automatically included for Prometheus Exporter
-require File.expand_path("../../config/environment", __dir__) unless defined? Rails
-
 module Collectors
   class GlobalPrometheusCollector < PrometheusExporter::Server::TypeCollector
+    SECONDS_PER_DAY = 86_400
+
     def type
       "locations_api_global"
     end
 
     def metrics
-      oldest_postcode = PrometheusExporter::Metric::Gauge.new("locations_api_postcode_oldest_timestamp_seconds", "Timestamp of update of oldest postcode served")
-      oldest_postcode.observe(get_oldest_postcode_served)
+      oldest_os_places_postcode = PrometheusExporter::Metric::Gauge.new("locations_api_oldest_os_places_postcode_age_days", "Days since last update of oldest postcode OS Places postcode")
+      oldest_os_places_postcode.observe(get_oldest_postcode / SECONDS_PER_DAY)
 
-      [oldest_postcode]
+      [oldest_os_places_postcode]
     end
 
   private
 
-    def token_expiry_info
+    def get_oldest_postcode
       # Cache metric to prevent needless expensive calls to the database
-      Rails.cache.fetch("token_expiry_info", expires_in: 1.hour) do
-        ApiUser.all.flat_map do |user|
-          user.authorisations.where(revoked_at: nil).map do |token|
-            {
-              expires_at: token.expires_at.to_i,
-              api_user: user.email,
-              application_name: token.application.name.parameterize,
-            }
-          end
-        end
+      Rails.cache.fetch("metrics:oldest_postcode", expires_in: 1.hour) do
+        (Time.zone.now.to_i - Postcode.os_places.order(updated_at: :asc).first.updated_at.to_i)
       end
     end
   end
-end
+end

spec/lib/collectors/global_prometheus_collector_spec.rb

@@ -0,0 +1,26 @@
+require "spec_helper"
+
+RSpec.describe Collectors::GlobalPrometheusCollector do
+  before do
+    @collector = Collectors::GlobalPrometheusCollector.new
+    Rails.cache.delete("metrics:oldest_postcode")
+  end
+
+  describe "#type" do
+    it "has the correct value" do
+      expect(@collector.type).to eq("locations_api_global")
+    end
+  end
+
+  describe "#metrics" do
+    before do
+      @postcode = Postcode.create!(postcode: "E18QS", updated_at: Time.zone.now - 2.days)
+    end
+
+    it "records the time in days since the oldest postcode's update timestamp" do
+      metrics = @collector.metrics
+
+      expect(metrics.first.data.first.last).to eq(2)
+    end
+  end
+end