Geo: Log to geo.log when the Log Cursor skips an event

This commit adds messages such as the following:

{
   "severity":"INFO",
   "time":"2018-07-11T20:43:04.505Z",
   "pid":53975,
   "class":"Gitlab::Geo::LogCursor::Daemon",
   "message":"Skipped event",
   "log_event_id":1,
   "event_id":1,
   "event_type":"Geo::RepositoryUpdatedEvent",
   "project_id":1,
   "cursor_delay_s":0.077
}

This will help diagnose whether the cursor is actually skipping events.

ee/changelogs/unreleased/sh-geo-log-skipped-events.yml

+---
+title: 'Geo: Log to geo.log when the Log Cursor skips an event'
+merge_request:
+author:
+type: other

ee/lib/gitlab/geo/log_cursor/daemon.rb

@@ -45,7 +45,10 @@ module Gitlab
 
         def handle_events(batch)
           batch.each do |event_log|
-            next unless can_replay?(event_log)
+            unless can_replay?(event_log)
+              logger.event_info(event_log.created_at, 'Skipped event', event_data(event_log))
+              next
+            end
 
             begin
               event = event_log.event
@@ -105,6 +108,15 @@ module Gitlab
         def log_level
           options[:debug] ? :debug : Rails.logger.level
         end
+
+        def event_data(event_log)
+          {
+            event_log_id: event_log.id,
+            event_id: event_log.event.id,
+            event_type: event_log.event.class.name,
+            project_id: event_log.project_id
+          }
+        end
       end
     end
   end

ee/spec/lib/gitlab/geo/log_cursor/daemon_spec.rb

@@ -115,6 +115,22 @@ describe Gitlab::Geo::LogCursor::Daemon, :postgresql, :clean_gitlab_redis_shared
         daemon.run_once!
       end
 
+      it 'logs a message for skipped events' do
+        secondary.update!(selective_sync_type: 'namespaces', namespaces: [group_2])
+
+        expect(Gitlab::Geo::Logger).to receive(:info).with(hash_including(
+                                                             :pid,
+                                                             :cursor_delay_s,
+                                                             message: 'Skipped event',
+                                                             class: 'Gitlab::Geo::LogCursor::Daemon',
+                                                             event_log_id: event_log.id,
+                                                             event_id: repository_updated_event.id,
+                                                             event_type: 'Geo::RepositoryUpdatedEvent',
+                                                             project_id: project.id))
+
+        daemon.run_once!
+      end
+
       it 'does not replay events for projects that do not belong to selected shards to replicate' do
         secondary.update!(selective_sync_type: 'shards', selective_sync_shards: ['broken'])