Log if a host is offline due to replication lag

ee/changelogs/unreleased/load_balancing_log_lines.yml

+---
+title: Improve DB load balancing log to log host offline due to replication lag
+merge_request: 15995
+author:
+type: changed

ee/lib/gitlab/database/load_balancing/host.rb

@@ -78,6 +78,13 @@ module Gitlab
               db_host: @host,
               db_port: @port
             )
+          else
+            LoadBalancing::Logger.warn(
+              event: :host_offline,
+              message: 'Host is offline after replica status check',
+              db_host: @host,
+              db_port: @port
+            )
           end
 
           @online

ee/spec/lib/gitlab/database/load_balancing/host_spec.rb

@@ -124,6 +124,20 @@ describe Gitlab::Database::LoadBalancing::Host do
 
         expect(host).to be_online
       end
+
+      context 'and replica is not up to date' do
+        before do
+          expect(host).to receive(:replica_is_up_to_date?).and_return(false)
+        end
+
+        it 'marks the host offline' do
+          expect(Gitlab::Database::LoadBalancing::Logger).to receive(:warn)
+            .with(hash_including(event: :host_offline))
+            .and_call_original
+
+          expect(host).not_to be_online
+        end
+      end
     end
 
     context 'when the replica is not online' do