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

lib/middleware/request_tracker.rb

@@ -105,7 +105,7 @@ class Middleware::RequestTracker
     track_view &&= env_track_view || (request.get? && !request.xhr? && headers["Content-Type"] =~ /text\/html/)
     track_view = !!track_view
 
-    {
+    h = {
       status: status,
       is_crawler: helper.is_crawler?,
       has_auth_cookie: helper.has_auth_cookie?,
@@ -114,9 +114,16 @@ class Middleware::RequestTracker
       track_view: track_view,
       timing: timing,
       queue_seconds: env['REQUEST_QUEUE_SECONDS']
-    }.tap do |h|
-      h[:user_agent] = env['HTTP_USER_AGENT'] if h[:is_crawler]
+    }
+
+    if h[:is_crawler]
+      h[:user_agent] = env['HTTP_USER_AGENT']
     end
+
+    if cache = headers["X-Discourse-Cached"]
+      h[:cache] = cache
+    end
+    h
   end
 
   def log_request_info(env, result, info)

spec/components/middleware/request_tracker_spec.rb

@@ -272,6 +272,19 @@ describe Middleware::RequestTracker do
       Middleware::RequestTracker.unregister_detailed_request_logger(logger)
     end
 
+    it "can report data from anon cache" do
+      cache = Middleware::AnonymousCache.new(app([200, {}, ["i am a thing"]]))
+      tracker = Middleware::RequestTracker.new(cache)
+
+      uri = "/path?#{SecureRandom.hex}"
+      tracker.call(env("REQUEST_URI" => uri, "ANON_CACHE_DURATION" => 60))
+
+      expect(@data[:cache]).to eq("store")
+
+      tracker.call(env("REQUEST_URI" => uri, "ANON_CACHE_DURATION" => 60))
+      expect(@data[:cache]).to eq("true")
+    end
+
     it "can correctly log detailed data" do
 
       global_setting :enable_performance_http_headers, true