apm: don't record unknown url params in the apm.

Don't record unknown url params that don't come from our app. This
includes typos, url params from userscripts, and weird params from
broken bots, crawlers, or other unknown sources. Indexing too many
params can lead to a mapping explosion.

https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping.html

app/logical/danbooru_logger.rb

@@ -58,7 +58,10 @@ class DanbooruLogger
   end
 
   def self.request_params(request)
-    request.parameters.with_indifferent_access.except(:controller, :action)
+    request.parameters.with_indifferent_access.except(:controller, :action).reject do |key, value|
+      # exclude strange URL params that don't come from our app.
+      !key.match?(/\A[a-z._]+\z/) || key.match?(/\A_|_\z/)
+    end
   end
 
   def self.session_params(session)