Merge branch 'sh-move-sidekiq-exporter-logs' into 'master'

Move Sidekiq exporter logs to log/sidekiq_exporter.log

See merge request gitlab-org/gitlab-ce!18127

changelogs/unreleased/sh-move-sidekiq-exporter-logs.yml

+---
+title: Move Sidekiq exporter logs to log/sidekiq_exporter.log
+merge_request:
+author:
+type: other

doc/administration/logs.md

@@ -206,4 +206,12 @@ is populated whenever `gitlab-ctl reconfigure` is run manually or as part of an
 Reconfigure logs files are named according to the UNIX timestamp of when the reconfigure
 was initiated, such as `1509705644.log`
 
+## `sidekiq_exporter.log`
+
+If Prometheus metrics and the Sidekiq Exporter are both enabled, Sidekiq will
+start a Web server and listen to the defined port (default: 3807). Access logs
+will be generated in `/var/log/gitlab/gitlab-rails/sidekiq_exporter.log` for
+Omnibus GitLab packages or in `/home/git/gitlab/log/sidekiq_exporter.log` for
+installations from source.
+
 [repocheck]: repository_checks.md

lib/gitlab/metrics/sidekiq_metrics_exporter.rb

@@ -4,6 +4,8 @@ require 'prometheus/client/rack/exporter'
 module Gitlab
   module Metrics
     class SidekiqMetricsExporter < Daemon
+      LOG_FILENAME = File.join(Rails.root, 'log', 'sidekiq_exporter.log')
+
       def enabled?
         Gitlab::Metrics.metrics_folder_present? && settings.enabled
       end
@@ -17,7 +19,13 @@ module Gitlab
       attr_reader :server
 
       def start_working
-        @server = ::WEBrick::HTTPServer.new(Port: settings.port, BindAddress: settings.address)
+        logger = WEBrick::Log.new(LOG_FILENAME)
+        access_log = [
+          [logger, WEBrick::AccessLog::COMBINED_LOG_FORMAT]
+        ]
+
+        @server = ::WEBrick::HTTPServer.new(Port: settings.port, BindAddress: settings.address,
+                                            Logger: logger, AccessLog: access_log)
         server.mount "/", Rack::Handler::WEBrick, rack_app
         server.start
       end

spec/lib/gitlab/metrics/sidekiq_metrics_exporter_spec.rb

@@ -38,7 +38,9 @@ describe Gitlab::Metrics::SidekiqMetricsExporter do
 
             expect(::WEBrick::HTTPServer).to have_received(:new).with(
               Port: port,
-              BindAddress: address
+              BindAddress: address,
+              Logger: anything,
+              AccessLog: anything
             )
           end
         end