search: log extra search metadata to NewRelic.

When a user does a tag search, log a few more things, including the normalized
search string, the number of tags in the search string, and the number of results.

app/controllers/posts_controller.rb

@@ -12,6 +12,7 @@ class PostsController < ApplicationController
       tag_query = params[:tags] || params.dig(:post, :tags)
       @post_set = PostSets::Post.new(tag_query, params[:page], params[:limit], random: params[:random], format: params[:format])
       @posts = authorize @post_set.posts, policy_class: PostPolicy
+      @post_set.log!
       respond_with(@posts) do |format|
         format.atom
       end

app/logical/post_sets/post.rb

@@ -160,6 +160,24 @@ module PostSets
       end
     end
 
+    def search_stats
+      {
+        query: normalized_query.to_s,
+        count: post_count,
+        page: current_page,
+        limit: per_page,
+        term_count: normalized_query.terms.count,
+        tag_count: normalized_query.tags.count,
+        metatag_count: normalized_query.metatags.count,
+        censored_posts: censored_posts.count,
+        hidden_posts: hidden_posts.count,
+      }
+    end
+
+    def log!
+      DanbooruLogger.add_attributes("search", search_stats)
+    end
+
     concerning :TagListMethods do
       def related_tags
         if query.is_wildcard_search?