Expose gauges to report status of partitioning

The two metrics are being reported on a per table basis. This allows
monitoring systems to detect a situation where a partition wasn't
created over an extended period of time. This is potentially a
disastrous state if a relevant partition hasn't been created yet but is
needed to create new records.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/227353

app/workers/partition_creation_worker.rb

@@ -9,5 +9,7 @@ class PartitionCreationWorker
 
   def perform
     Gitlab::Database::Partitioning::PartitionCreator.new.create_partitions
+  ensure
+    Gitlab::Database::Partitioning::PartitionMonitoring.new.report_metrics
   end
 end

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -194,6 +194,15 @@ The following metrics are available:
 |:--------------------------------- |:--------- |:------------------------------------------------------------- |:-------------------------------------- |
 | `db_load_balancing_hosts`         | Gauge     | [12.3](https://gitlab.com/gitlab-org/gitlab/-/issues/13630)     | Current number of load balancing hosts |
 
+## Database partitioning metrics **(PREMIUM ONLY)**
+
+The following metrics are available:
+
+| Metric                            | Type      | Since                                                         | Description                                                       |
+|:--------------------------------- |:--------- |:------------------------------------------------------------- |:----------------------------------------------------------------- |
+| `db_partitions_present`           | Gauge     | [13.4](https://gitlab.com/gitlab-org/gitlab/-/issues/227353)  | Number of database partitions present                             |
+| `db_partitions_missing`           | Gauge     | [13.4](https://gitlab.com/gitlab-org/gitlab/-/issues/227353)  | Number of database partitions currently expected, but not present |
+
 ## Connection pool metrics
 
 These metrics record the status of the database

lib/gitlab/database/partitioning/partition_monitoring.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Database
+    module Partitioning
+      class PartitionMonitoring
+        attr_reader :models
+
+        def initialize(models = PartitionCreator.models)
+          @models = models
+        end
+
+        def report_metrics
+          models.each do |model|
+            strategy = model.partitioning_strategy
+
+            gauge_present.set({ table: model.table_name }, strategy.current_partitions.size)
+            gauge_missing.set({ table: model.table_name }, strategy.missing_partitions.size)
+          end
+        end
+
+        private
+
+        def gauge_present
+          @gauge_present ||= Gitlab::Metrics.gauge(:db_partitions_present, 'Number of database partitions present')
+        end
+
+        def gauge_missing
+          @gauge_missing ||= Gitlab::Metrics.gauge(:db_partitions_missing, 'Number of database partitions currently expected, but not present')
+        end
+      end
+    end
+  end
+end

spec/lib/gitlab/database/partitioning/partition_monitoring_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::Database::Partitioning::PartitionMonitoring do
+  describe '#report_metrics' do
+    subject { described_class.new(models).report_metrics }
+
+    let(:models) { [model] }
+    let(:model) { double(partitioning_strategy: partitioning_strategy, table_name: table) }
+    let(:partitioning_strategy) { double(missing_partitions: missing_partitions, current_partitions: current_partitions) }
+    let(:table) { "some_table" }
+
+    let(:missing_partitions) do
+      [double]
+    end
+
+    let(:current_partitions) do
+      [double, double]
+    end
+
+    it 'reports number of present partitions' do
+      subject
+
+      expect(Gitlab::Metrics.registry.get(:db_partitions_present).get({ table: table })).to eq(current_partitions.size)
+    end
+
+    it 'reports number of missing partitions' do
+      subject
+
+      expect(Gitlab::Metrics.registry.get(:db_partitions_missing).get({ table: table })).to eq(missing_partitions.size)
+    end
+  end
+end

spec/workers/partition_creation_worker_spec.rb

@@ -4,16 +4,26 @@ require "spec_helper"
 
 RSpec.describe PartitionCreationWorker do
   describe '#perform' do
-    let(:creator) { double(create_partitions: nil) }
+    subject { described_class.new.perform }
+
+    let(:creator) {  instance_double('PartitionCreator', create_partitions: nil) }
+    let(:monitoring) { instance_double('PartitionMonitoring', report_metrics: nil) }
 
     before do
       allow(Gitlab::Database::Partitioning::PartitionCreator).to receive(:new).and_return(creator)
+      allow(Gitlab::Database::Partitioning::PartitionMonitoring).to receive(:new).and_return(monitoring)
     end
 
     it 'delegates to PartitionCreator' do
       expect(creator).to receive(:create_partitions)
 
-      described_class.new.perform
+      subject
+    end
+
+    it 'reports partition metrics' do
+      expect(monitoring).to receive(:report_metrics)
+
+      subject
     end
   end
 end