Use module_function in Banzai::Renderer

Using `extend self` prevents GitLab Performance Monitoring from being
able to track class methods.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/23347

CHANGELOG

@@ -14,6 +14,7 @@ v 8.13.0 (unreleased)
   - Add an example for testing a phoenix application with Gitlab CI in the docs (Manthan Mallikarjun)
   - Updating verbiage on git basics to be more intuitive
   - Clarify documentation for Runners API (Gennady Trafimenkov)
+  - The instrumentation for Banzai::Renderer has been restored
   - Change user & group landing page routing from /u/:username to /:username
   - Prevent running GfmAutocomplete setup for each diff note !6569
   - Added documentation for .gitattributes files

lib/banzai/renderer.rb

 module Banzai
   module Renderer
-    extend self
+    module_function
 
     # Convert a Markdown String into an HTML-safe String of HTML
     #
@@ -141,8 +141,6 @@ module Banzai
       end.html_safe
     end
 
-    private
-
     def cacheless_render(text, context = {})
       Gitlab::Metrics.measure(:banzai_cacheless_render) do
         result = render_result(text, context)