Move Sidekiq exporter logs to log/sidekiq_exporter.log

The Sidekiq exporter logs were mixing with the normal Sidekiq logs. In order
to support structured logging in Sidekiq, we either need to split this data
out or convert the exporter to produce structured logs. Since Sidekiq job
processing is fundamentally different information from Web server traffic,
it seems cleaner to move the metrics traffic into a separate file, where they
can be parsed by a different filter if needed.

Relates to #20060

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