Convert Sidekiq scheduled_at to a timestamp field

This makes it consistent with other timestamp fields that we log

lib/gitlab/sidekiq_logging/json_formatter.rb

@@ -6,7 +6,7 @@ require 'json'
 module Gitlab
   module SidekiqLogging
     class JSONFormatter
-      TIMESTAMP_FIELDS = %w[created_at enqueued_at started_at retried_at failed_at completed_at].freeze
+      TIMESTAMP_FIELDS = %w[created_at scheduled_at enqueued_at started_at retried_at failed_at completed_at].freeze
 
       def call(severity, timestamp, progname, data)
         output = {

spec/lib/gitlab/sidekiq_logging/json_formatter_spec.rb

@@ -17,6 +17,7 @@ RSpec.describe Gitlab::SidekiqLogging::JSONFormatter do
         'class' => 'PostReceive',
         'bar' => 'test',
         'created_at' => timestamp,
+        'scheduled_at' => timestamp,
         'enqueued_at' => timestamp,
         'started_at' => timestamp,
         'retried_at' => timestamp,
@@ -31,6 +32,7 @@ RSpec.describe Gitlab::SidekiqLogging::JSONFormatter do
           'severity' => 'INFO',
           'time' => timestamp_iso8601,
           'created_at' => timestamp_iso8601,
+          'scheduled_at' => timestamp_iso8601,
           'enqueued_at' => timestamp_iso8601,
           'started_at' => timestamp_iso8601,
           'retried_at' => timestamp_iso8601,