Enable improved_mr_merged_at_queries FF by default

This change improves the query timings for the Productivity Analytics
and Merge Request analytics features.

app/finders/concerns/merged_at_filter.rb

@@ -12,7 +12,7 @@ module MergedAtFilter
     mr_metrics_scope = mr_metrics_scope.merged_before(merged_before) if merged_before.present?
 
     scope = items.joins(:metrics).merge(mr_metrics_scope)
-    scope = target_project_id_filter_on_metrics(scope) if Feature.enabled?(:improved_mr_merged_at_queries)
+    scope = target_project_id_filter_on_metrics(scope) if Feature.enabled?(:improved_mr_merged_at_queries, default_enabled: true)
     scope
   end
   # rubocop: enable CodeReuse/ActiveRecord

changelogs/unreleased/enable_improved_mr_merged_at_queries_ff_by_default.yml

+---
+title: Improve Productivity Analytics and Merge Request Analytics database queries
+merge_request: 40838
+author:
+type: performance

config/feature_flags/development/improved_mr_merged_at_queries.yml

@@ -4,4 +4,4 @@ introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/39329
 rollout_issue_url: 
 group: group::analytics
 type: development
-default_enabled: false
+default_enabled: true