Add system note when time estimate for Issue/MergeRequest is changed.

app/services/issuable_base_service.rb

@@ -36,6 +36,11 @@ class IssuableBaseService < BaseService
     end
   end
 
+  def create_time_estimate_note(issuable)
+    SystemNoteService.change_time_estimate(
+      issuable, issuable.project, current_user, issuable.time_estimate)
+  end
+
   def filter_params(issuable_ability_name = :issue)
     filter_assignee
     filter_milestone

app/services/issues/update_service.rb

@@ -28,6 +28,10 @@ module Issues
         create_confidentiality_note(issue)
       end
 
+      if issue.previous_changes.include?('time_estimate')
+        create_time_estimate_note(issue)
+      end
+
       added_labels = issue.labels - old_labels
       if added_labels.present?
         notification_service.relabeled_issue(issue, added_labels, current_user)

app/services/system_note_service.rb

@@ -111,6 +111,26 @@ module SystemNoteService
     create_note(noteable: noteable, project: project, author: author, note: body)
   end
 
+  # Called when the estimated time of a Noteable is changed
+  #
+  # noteable  - Noteable object
+  # project   - Project owning noteable
+  # author    - User performing the change
+  # milestone - Milestone being assigned, or nil
+  #
+  # Example Note text:
+  #
+  #   "Changed estimate of this issue to 3d 5h"
+  #
+  # Returns the created Note object
+
+  def change_time_estimate(noteable, project, author, time_estimate)
+    parsed_time = ChronicDuration.output(time_estimate, format: :short)
+    body = "Changed estimate of this #{noteable.to_ability_name} to #{parsed_time}"
+
+    create_note(noteable: noteable, project: project, author: author, note: body)
+  end
+
   # Called when the status of a Noteable is changed
   #
   # noteable - Noteable object