Only log host online events if host was offline

Host online events were previously logged every time a host became
online. This produced a huge amount of log volume.

lib/gitlab/database/load_balancing/host.rb

@@ -119,16 +119,19 @@ def offline!
         def online?
           return @online unless check_replica_status?
+          was_online = @online
           refresh_status
-          if @online
+          # Log that the host came back online if it was previously offline
+          if @online && !was_online
             ::Gitlab::Database::LoadBalancing::Logger.info(
               event: :host_online,
               message: 'Host is online after replica status check',
               db_host: @host,
               db_port: @port
             )
-          else
+          # Always log if the host goes offline
+          elsif !@online
             ::Gitlab::Database::LoadBalancing::Logger.warn(
               event: :host_offline,
               message: 'Host is offline after replica status check',

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

@@ -2,7 +2,7 @@
 require 'spec_helper'
-RSpec.describe Gitlab::Database::LoadBalancing::Host do
+RSpec.describe Gitlab::Database::LoadBalancing::Host, feature_category: :database do
   let(:load_balancer) do
     Gitlab::Database::LoadBalancing::LoadBalancer
       .new(Gitlab::Database::LoadBalancing::Configuration.new(ActiveRecord::Base))
@@ -124,13 +124,36 @@ def wrapped_exception(wrapper, original)
       end
       it 'refreshes the status' do
-        expect(Gitlab::Database::LoadBalancing::Logger).to receive(:info)
-          .with(hash_including(event: :host_online))
-          .and_call_original
-
         expect(host).to be_online
       end
+      context 'and the host was previously online' do
+        # Hosts are online by default
+
+        it 'does not log the online event' do
+          expect(Gitlab::Database::LoadBalancing::Logger)
+            .not_to receive(:info)
+            .with(hash_including(event: :host_online))
+
+          expect(host).to be_online
+        end
+      end
+
+      context 'and the host was previously offline' do
+        before do
+          host.offline!
+        end
+
+        it 'logs the online event' do
+          expect(Gitlab::Database::LoadBalancing::Logger)
+            .to receive(:info)
+            .with(hash_including(event: :host_online))
+            .and_call_original
+
+          expect(host).to be_online
+        end
+      end
+
       context 'and replica is not up to date' do
         before do
           expect(host).to receive(:replica_is_up_to_date?).and_return(false)