FEATURE: anon cache reports data to loggers

This allows custom plugins such as prometheus exporter to log how many
requests are stored in the anon cache vs used by the anon cache.

This metric allows us to fine tune cache behaviors

lib/middleware/anonymous_cache.rb

@@ -182,6 +182,8 @@ module Middleware
 
           $redis.setex(cache_key_body,  cache_duration, parts.join)
           $redis.setex(cache_key_other, cache_duration, [status, headers_stripped].to_json)
+
+          headers["X-Discourse-Cached"] = "store"
         else
           parts = response
         end