Allow overriding timezone experiment per category

This allows enabling or disabling the timezone experiment per
category.

It disabled the experiment for the database category, since the group
of maintainers too small.

lib/gitlab/danger/roulette.rb

@@ -8,6 +8,10 @@ module Gitlab
       ROULETTE_DATA_URL = 'https://gitlab-org.gitlab.io/gitlab-roulette/roulette.json'
       HOURS_WHEN_PERSON_CAN_BE_PICKED = (6..14).freeze
 
+      INCLUDE_TIMEZONE_FOR_CATEGORY = {
+        database: false
+      }.freeze
+
       Spin = Struct.new(:category, :reviewer, :maintainer, :optional_role)
 
       # Assigns GitLab team members to be reviewer and maintainer
@@ -26,7 +30,9 @@ module Gitlab
         canonical_branch_name = canonical_branch_name(branch_name)
 
         spin_per_category = categories.each_with_object({}) do |category, memo|
-          memo[category] = spin_for_category(team, project, category, canonical_branch_name, timezone_experiment: timezone_experiment)
+          including_timezone = INCLUDE_TIMEZONE_FOR_CATEGORY.fetch(category, timezone_experiment)
+
+          memo[category] = spin_for_category(team, project, category, canonical_branch_name, timezone_experiment: including_timezone)
         end
 
         spin_per_category.map do |category, spin|

spec/lib/gitlab/danger/roulette_spec.rb

@@ -194,6 +194,28 @@ RSpec.describe Gitlab::Danger::Roulette do
           end
         end
       end
+
+      context 'when change includes a category with timezone disabled' do
+        let(:categories) { [:backend] }
+
+        before do
+          stub_const("#{described_class}::INCLUDE_TIMEZONE_FOR_CATEGORY", backend: false)
+        end
+
+        it 'assigns backend reviewer and maintainer' do
+          expect(spins).to contain_exactly(matching_spin(:backend, reviewer: engineering_productivity_reviewer, maintainer: backend_maintainer))
+        end
+
+        context 'when teammate is not in a good timezone' do
+          before do
+            backend_maintainer[:tz_offset_hours] = 5.0
+          end
+
+          it 'assigns backend reviewer and maintainer' do
+            expect(spins).to contain_exactly(matching_spin(:backend, reviewer: engineering_productivity_reviewer, maintainer: backend_maintainer))
+          end
+        end
+      end
     end
   end