Improve ETag caching for issue discussions

This removes the issue_discussions_http_cache feature flag. This allows
us to avoid JSON serialization when the ETag matches.

This is working as expected and has been tested on GitLab.com for 3
weeks already.

Changelog: performance

app/controllers/concerns/issuable_actions.rb

@@ -160,7 +160,7 @@ module IssuableActions
 
     if issuable.is_a?(MergeRequest)
       render_cached(discussions, with: discussion_serializer, cache_context: -> (_) { discussion_cache_context }, context: self)
-    elsif issuable.is_a?(Issue) && Feature.enabled?(:issue_discussions_http_cache, default_enabled: :yaml)
+    elsif issuable.is_a?(Issue)
       render json: discussion_serializer.represent(discussions, context: self) if stale?(etag: [discussion_cache_context, discussions])
     else
       render json: discussion_serializer.represent(discussions, context: self)

config/feature_flags/development/issue_discussions_http_cache.yml

----
-name: issue_discussions_http_cache
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/72589
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/343309
-milestone: '14.5'
-type: development
-group: group::project management
-default_enabled: false

spec/requests/projects/issues/discussions_spec.rb

@@ -14,6 +14,7 @@ RSpec.describe 'issue discussions' do
       project.add_maintainer(user)
     end
 
+    context 'HTTP caching' do
       def get_discussions
         get discussions_namespace_project_issue_path(namespace_id: project.namespace, project_id: project, id: issue.iid), headers: {
           'If-None-Match' => @etag
@@ -28,17 +29,6 @@ RSpec.describe 'issue discussions' do
         get_discussions
       end
 
-    shared_examples 'cache miss' do
-      it 'returns 200 and serializes JSON' do
-        expect(DiscussionSerializer).to receive(:new).and_call_original
-
-        get_discussions
-
-        expect(response).to have_gitlab_http_status(:ok)
-      end
-    end
-
-    shared_examples 'cache hit' do
       it 'returns 304 without serializing JSON' do
         expect(DiscussionSerializer).not_to receive(:new)
 
@@ -46,22 +36,16 @@ RSpec.describe 'issue discussions' do
 
         expect(response).to have_gitlab_http_status(:not_modified)
       end
-    end
 
-    context 'when issue_discussions_http_cache is disabled' do
-      before do
-        stub_feature_flags(issue_discussions_http_cache: false)
-      end
+      shared_examples 'cache miss' do
+        it 'returns 200 and serializes JSON' do
+          expect(DiscussionSerializer).to receive(:new).and_call_original
 
-      it_behaves_like 'cache miss'
-    end
+          get_discussions
 
-    context 'when issue_discussions_http_cache is enabled' do
-      before do
-        stub_feature_flags(issue_discussions_http_cache: true)
+          expect(response).to have_gitlab_http_status(:ok)
+        end
       end
-
-      it_behaves_like 'cache hit'
 
       context 'when user role changes' do
         before do