Globally enable better generic metrics comparison

Use merge base pipeline instead of diff base pipeline.
When using pipelines for merged results, the comparison
will be truer to the semantic diff of the merge request.

Changelog: changed

app/models/merge_request.rb

@@ -37,7 +37,7 @@ class MergeRequest < ApplicationRecord
   SORTING_PREFERENCE_FIELD = :merge_requests_sort
 
   ALLOWED_TO_USE_MERGE_BASE_PIPELINE_FOR_COMPARISON = {
-    'Ci::CompareMetricsReportsService'     => ->(project) { ::Gitlab::Ci::Features.merge_base_pipeline_for_metrics_comparison?(project) },
+    'Ci::CompareMetricsReportsService'     => ->(project) { true },
     'Ci::CompareCodequalityReportsService' => ->(project) { true }
   }.freeze
 

config/feature_flags/development/merge_base_pipeline_for_metrics_comparison.yml

----
-name: merge_base_pipeline_for_metrics_comparison
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/61282
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/330809
-milestone: '13.12'
-type: development
-group: group::testing
-default_enabled: false

lib/gitlab/ci/features.rb

@@ -18,10 +18,6 @@ module Gitlab
         Feature.enabled?(:ci_pipeline_status_omit_commit_sha_in_cache_key, project, default_enabled: true)
       end
 
-      def self.merge_base_pipeline_for_metrics_comparison?(project)
-        Feature.enabled?(:merge_base_pipeline_for_metrics_comparison, project, default_enabled: :yaml)
-      end
-
       # NOTE: The feature flag `disallow_to_create_merge_request_pipelines_in_target_project`
       # is a safe switch to disable the feature for a particular project when something went wrong,
       # therefore it's not supposed to be enabled by default.

spec/models/merge_request_spec.rb

@@ -3880,14 +3880,6 @@ RSpec.describe MergeRequest, factory_default: :keep do
       let(:service_class) { 'Ci::CompareMetricsReportsService' }
 
       it { is_expected.to be_truthy }
-
-      context 'with the metrics report flag disabled' do
-        before do
-          stub_feature_flags(merge_base_pipeline_for_metrics_comparison: false)
-        end
-
-        it { is_expected.to be_falsey }
-      end
     end
 
     context 'when service class is Ci::CompareCodequalityReportsService' do