Stop recording connection pool metrics for load balancing hosts

The cardinality of these is simply too high: for a single (Rails) host
on GitLab.com, we have 384 elements due to a combination of:

1. Puma threads.
2. Load balancing hosts.
3. Load balancing ports.

Realistically, the saturation for a load balancing connection pool
should never be higher than for ActiveRecord::Base anyway.

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -193,8 +193,6 @@ They all have these labels:
     1. `ActiveRecord::Base` is the main database connection.
     1. `Geo::TrackingBase` is the connection to the Geo tracking database, if
        enabled.
-    1. `Gitlab::Database::LoadBalancing::Host` is a connection used by [database
-       load balancing](../../database_load_balancing.md), if enabled.
 1. `host` - the host name used to connect to the database.
 1. `port` - the port used to connect to the database.
 

ee/changelogs/unreleased/stop-recording-load-balancing-metrics.yml

+---
+title: Stop recording connection pool metrics for load balancing hosts
+merge_request: 33749
+author:
+type: removed

ee/lib/ee/gitlab/metrics/samplers/database_sampler.rb

@@ -6,15 +6,12 @@ module EE
       module Samplers
         module DatabaseSampler
           extend ::Gitlab::Utils::Override
-          include ::Gitlab::Utils::StrongMemoize
 
           private
 
           override :host_stats
           def host_stats
-            super
-              .concat(geo_connection_stats)
-              .concat(load_balancing_connection_stats)
+            super.concat(geo_connection_stats)
           end
 
           def geo_connection_stats
@@ -22,23 +19,6 @@ module EE
 
             [{ labels: labels_for_class(Geo::TrackingBase), stats: Geo::TrackingBase.connection_pool.stat }]
           end
-
-          def load_balancing_connection_stats
-            return [] unless load_balancing_enabled?
-
-            ActiveRecord::Base.connection.load_balancer.host_list.hosts.map do |host|
-              {
-                labels: { host: host.host, port: host.port, class: 'Gitlab::Database::LoadBalancing::Host' },
-                stats: host.pool.stat
-              }
-            end
-          end
-
-          def load_balancing_enabled?
-            strong_memoize(:load_balancing_enabled) do
-              ::Gitlab::Database::LoadBalancing.enable?
-            end
-          end
         end
       end
     end

ee/spec/lib/ee/gitlab/metrics/samplers/database_sampler_spec.rb

@@ -45,47 +45,5 @@ describe Gitlab::Metrics::Samplers::DatabaseSampler do
         end
       end
     end
-
-    context 'for Gitlab::Database::LoadBalancing::Host' do
-      let(:labels) { { class: 'Gitlab::Database::LoadBalancing::Host' } }
-
-      context 'when database load balancing is enabled' do
-        let(:hosts) { %w[secondary-1 secondary-2] }
-        let(:proxy) { ::Gitlab::Database::LoadBalancing::ConnectionProxy.new(hosts) }
-
-        before do
-          allow(ActiveRecord::Base).to receive(:connection).and_return(proxy)
-          allow(::Gitlab::Database::LoadBalancing).to receive(:enable?).and_return(true)
-        end
-
-        it 'samples connection pool statistics for all hosts' do
-          hosts.each do |host|
-            expected_labels = a_hash_including(host: host, **labels)
-
-            expect(subject.metrics[:size]).to receive(:set).with(expected_labels, a_value >= 1)
-            expect(subject.metrics[:connections]).to receive(:set).with(expected_labels, a_value >= 0)
-            expect(subject.metrics[:busy]).to receive(:set).with(expected_labels, a_value >= 0)
-            expect(subject.metrics[:dead]).to receive(:set).with(expected_labels, a_value >= 0)
-            expect(subject.metrics[:waiting]).to receive(:set).with(expected_labels, a_value >= 0)
-          end
-
-          subject.sample
-        end
-      end
-
-      context 'when database load balancing is not enabled' do
-        it 'records no samples' do
-          expect(subject.metrics[:size]).not_to receive(:set).with(a_hash_including(labels), anything)
-
-          subject.sample
-        end
-
-        it 'still records samples for other connections' do
-          expect(subject.metrics[:size]).to receive(:set)
-
-          subject.sample
-        end
-      end
-    end
   end
 end