Add cache_start_and_finish_as and update specs

lib/gitlab/usage/metrics/instrumentations/database_metric.rb

@@ -38,7 +38,11 @@ module Gitlab
               @column = column
             end
 
-            attr_reader :metric_operation, :metric_relation, :metric_start, :metric_finish, :column
+            def cache_start_and_finish_as(cache_key)
+              @cache_key = cache_key
+            end
+
+            attr_reader :metric_operation, :metric_relation, :metric_start, :metric_finish, :column, :cache_key
           end
 
           def value
@@ -87,7 +91,11 @@ module Gitlab
           end
 
           def get_or_cache_batch_ids
-            key_name = "metric_instrumentation/#{relation.table_name}"
+            key_name = if self.class.cache_key.present?
+                         "metric_instrumentation/#{self.class.cache_key}"
+                       else
+                         "metric_instrumentation/#{relation.table_name}"
+                       end
 
             start = Rails.cache.fetch("#{key_name}_minimum_id", expires_in: 1.day) do
               self.class.start

spec/lib/gitlab/usage/metrics/instrumentations/database_metric_spec.rb

@@ -31,5 +31,37 @@ RSpec.describe Gitlab::Usage::Metrics::Instrumentations::DatabaseMetric do
       expect(Rails.cache.read('metric_instrumentation/issues_minimum_id')).to eq(314)
       expect(Rails.cache.read('metric_instrumentation/issues_maximum_id')).to eq(949)
     end
+
+    context 'with start and finish not called' do
+      subject do
+        described_class.tap do |m|
+          m.relation { Issue }
+          m.operation :count
+        end.new(time_frame: 'all')
+      end
+
+      it 'calculates a correct result' do
+        expect(subject.value).to eq(3)
+      end
+    end
+
+    context 'with cache_start_and_finish_as called' do
+      subject do
+        described_class.tap do |m|
+          m.relation { Issue }
+          m.operation :count
+          m.start { m.relation.minimum(:id) }
+          m.finish { m.relation.maximum(:id) }
+          m.cache_start_and_finish_as :special_issue_count
+        end.new(time_frame: 'all')
+      end
+
+      it 'caches using the key name passed', :use_clean_rails_redis_caching do
+        subject.value
+
+        expect(Rails.cache.read('metric_instrumentation/special_issue_count_minimum_id')).to eq(314)
+        expect(Rails.cache.read('metric_instrumentation/special_issue_count_maximum_id')).to eq(949)
+      end
+    end
   end
 end