Introduce API content type feature flag

When enabled APi will use json as default content type.
Requires application restart to take effect.
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Signed-off-by: Dmytro Zaporozhets <dzaporozhets@gitlab.com>

config/feature_flags/development/api_json_content_type.yml

+---
+name: api_json_content_type
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/42229
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/270067
+group: group::ecosystem
+type: development
+default_enabled: false

lib/api/api.rb

@@ -115,7 +115,14 @@ module API
 
     format :json
     formatter :json, Gitlab::Json::GrapeFormatter
-    content_type :txt, "text/plain"
+
+    # There is a small chance some users depend on the old behavior.
+    # We this change under a feature flag to see if affects GitLab.com users.
+    if Feature.enabled?(:api_json_content_type)
+      content_type :json, 'application/json'
+    else
+      content_type :txt, 'text/plain'
+    end
 
     # Ensure the namespace is right, otherwise we might load Grape::API::Helpers
     helpers ::API::Helpers

lib/api/ci/runner.rb

@@ -5,6 +5,8 @@ module API
     class Runner < Grape::API::Instance
       helpers ::API::Helpers::Runner
 
+      content_type :txt, 'text/plain'
+
       resource :runners do
         desc 'Registers a new Runner' do
           success Entities::RunnerRegistrationDetails

lib/api/go_proxy.rb

@@ -9,6 +9,8 @@ module API
 
     MODULE_VERSION_REQUIREMENTS = { module_version: MODULE_VERSION_REGEX }.freeze
 
+    content_type :txt, 'text/plain'
+
     before { require_packages_enabled! }
 
     helpers do

lib/api/repositories.rb

@@ -6,6 +6,8 @@ module API
   class Repositories < Grape::API::Instance
     include PaginationParams
 
+    content_type :txt, 'text/plain'
+
     helpers ::API::Helpers::HeadersHelpers
 
     before { authorize! :download_code, user_project }