Create dedicated log file for Elasticsearch integration

Dedicated log files for different parts of the system will help us and
our users debugging parts of the system that obviously relate to this
feature.

In future we should add even more logging to this file especially around
indexing events and any exceptions we're catching in sidekiq or web.

doc/administration/logs.md

@@ -393,6 +393,23 @@ It is stored at:
 - `/var/log/gitlab/gitlab-rails/database_load_balancing.log` for Omnibus GitLab packages.
 - `/home/git/gitlab/log/database_load_balancing.log` for installations from source.
 
+## `elasticsearch.log`
+
+Introduced in GitLab 12.6. This file lives in
+`/var/log/gitlab/gitlab-rails/elasticsearch.log` for Omnibus GitLab
+packages or in `/home/git/gitlab/log/elasticsearch.log` for installations
+from source.
+
+It logs information related to the Elasticsearch Integration including
+errors during indexing or searching Elasticsearch.
+
+Each line contains a JSON line that can be ingested by Elasticsearch, Splunk,
+etc. For example:
+
+```json
+{"severity":"DEBUG","time":"2019-10-17T06:23:13.227Z","correlation_id":null,"message":"redacted_search_result","class_name":"Milestone","id":2,"ability":"read_milestone","current_user_id":2,"query":"project"}
+```
+
 [repocheck]: repository_checks.md
 [Rack Attack]: ../security/rack_attack.md
 [Rate Limit]: ../user/admin_area/settings/rate_limits_on_raw_endpoints.md

ee/app/services/ee/search_service.rb

@@ -52,7 +52,7 @@ module EE
     private
 
     def logger
-      @logger ||= ::Gitlab::ProjectServiceLogger.build
+      @logger ||= ::Gitlab::Elasticsearch::Logger.build
     end
   end
 end

lib/gitlab/elasticsearch/logger.rb

+# frozen_string_literal: true
+
+module Gitlab
+  module Elasticsearch
+    class Logger < ::Gitlab::JsonLogger
+      def self.file_name_noext
+        'elasticsearch'
+      end
+    end
+  end
+end