Enable resource change event tracking

This sets the feature flag to be enabled by default. Instead of system
notes, event records would be created in the backend.

State, milestone and weight changes will no longer show up in the Notes
API. These can be accessed using their respective resource events APIs.

app/services/issuable/common_system_notes_service.rb

@@ -108,7 +108,7 @@ module Issuable
     end
 
     def milestone_changes_tracking_enabled?
-      ::Feature.enabled?(:track_resource_milestone_change_events, issuable.project)
+      ::Feature.enabled?(:track_resource_milestone_change_events, issuable.project, default_enabled: true)
     end
 
     def create_due_date_note

app/services/system_notes/issuables_service.rb

@@ -341,7 +341,7 @@ module SystemNotes
 
     def state_change_tracking_enabled?
       noteable.respond_to?(:resource_state_events) &&
-        ::Feature.enabled?(:track_resource_state_change_events, noteable.project)
+        ::Feature.enabled?(:track_resource_state_change_events, noteable.project, default_enabled: true)
     end
   end
 end

changelogs/unreleased/229147-enable-milestone-weight-events-feature-flag.yml

+---
+title: Track milestone and state changes in issues / MRs using resource events
+merge_request: 36936
+author:
+type: changed

doc/api/notes.md

@@ -7,7 +7,19 @@ Notes are comments on:
 - Merge requests
 - Epics **(ULTIMATE)**
 
-This includes system notes, which are notes about changes to the object (for example, when a milestone changes, there will be a corresponding system note). Label notes are not part of this API, but recorded as separate events in [resource label events](resource_label_events.md).
+This includes system notes, which are notes about changes to the object (for example, when an
+assignee changes, there will be a corresponding system note).
+
+## Resource events
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/38096) in GitLab 13.3 for state, milestone, and weight events.
+
+Some system notes are not part of this API, but are recorded as separate events:
+
+- [Resource label events](resource_label_events.md)
+- [Resource state events](resource_state_events.md)
+- [Resource milestone events](resource_milestone_events.md)
+- [Resource weight events](resource_weight_events.md) **(STARTER)**
 
 ## Notes pagination
 

ee/app/services/ee/issuable/common_system_notes_service.rb

@@ -58,7 +58,7 @@ module EE
       end
 
       def weight_changes_tracking_enabled?
-        !issuable.is_a?(Epic) && ::Feature.enabled?(:track_issue_weight_change_events, issuable.project)
+        !issuable.is_a?(Epic) && ::Feature.enabled?(:track_issue_weight_change_events, issuable.project, default_enabled: true)
       end
     end
   end

ee/changelogs/unreleased/229147-enable-milestone-weight-events-feature-flag.yml

+---
+title: Track issue weight changes using resource events
+merge_request: 36936
+author:
+type: changed