Add prometheus collector for oldest OS Places postcode

config/initializers/prometheus.rb

@@ -1,2 +1,4 @@
 require "govuk_app_config/govuk_prometheus_exporter"
-GovukPrometheusExporter.configure
+require "collectors/global_prometheus_collector"
+
+GovukPrometheusExporter.configure(collectors: [Collectors::GlobalPrometheusCollector])

lib/collectors/global_prometheus_collector.rb

@@ -0,0 +1,28 @@
+require "prometheus_exporter"
+require "prometheus_exporter/server"
+
+module Collectors
+  class GlobalPrometheusCollector < PrometheusExporter::Server::TypeCollector
+    SECONDS_PER_DAY = 86_400
+
+    def type
+      "locations_api_global"
+    end
+
+    def metrics
+      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_os_places_postcode]
+    end
+
+  private
+
+    def get_oldest_postcode
+      # Cache metric to prevent needless expensive calls to the database
+      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

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