Detect unsupported keyset queries with Prometheus

lib/gitlab/graphql/pagination/keyset/generic_keyset_pagination.rb

@@ -73,10 +73,25 @@ module Gitlab
             strong_memoize(:generic_keyset_pagination_items) do
               rebuilt_items_with_keyset_order, success = Gitlab::Pagination::Keyset::SimpleOrderBuilder.build(original_items)
 
-              success ? rebuilt_items_with_keyset_order : original_items
+              if success
+                rebuilt_items_with_keyset_order
+              else
+                if original_items.is_a?(ActiveRecord::Relation)
+                  old_keyset_pagination_usage.increment({ model: original_items.model.to_s })
+                end
+
+                original_items
               end
             end
           end
+
+          def old_keyset_pagination_usage
+            @old_keyset_pagination_usage ||= Gitlab::Metrics.counter(
+              :old_keyset_pagination_usage,
+              'The number of times the old keyset pagination code was used'
+            )
+          end
+        end
       end
     end
   end

spec/lib/gitlab/graphql/pagination/keyset/connection_spec.rb

@@ -77,6 +77,17 @@ RSpec.describe Gitlab::Graphql::Pagination::Keyset::Connection do
       expect(decoded_cursor(cursor)).to eq('id' => project.id.to_s)
     end
 
+    context 'when SimpleOrderBuilder cannot build keyset paginated query' do
+      it 'increments the `old_keyset_pagination_usage` counter', :prometheus do
+        expect(Gitlab::Pagination::Keyset::SimpleOrderBuilder).to receive(:build).and_return([false, nil])
+
+        decoded_cursor(cursor)
+
+        counter = Gitlab::Metrics.registry.get(:old_keyset_pagination_usage)
+        expect(counter.get(model: 'Project')).to eq(1)
+      end
+    end
+
     context 'when an order is specified' do
       let(:nodes) { Project.order(:updated_at) }