Duing application init connection can be assigned to `NullPool`

This appears to be a case for lazy init of performance bar
that in some circumstances results in a exception raised
as `db_config` is undefined

lib/gitlab/database/load_balancing.rb

@@ -133,6 +133,11 @@ module Gitlab
         # as this is dependent on a execution context
         return ROLE_UNKNOWN if connection.is_a?(ConnectionProxy)
 
+        # During application init we might receive `NullPool`
+        return ROLE_UNKNOWN unless connection.respond_to?(:pool) &&
+          connection.pool.respond_to?(:db_config) &&
+          connection.pool.db_config.respond_to?(:name)
+
         if connection.pool.db_config.name.ends_with?(LoadBalancer::REPLICA_SUFFIX)
           ROLE_REPLICA
         else

spec/lib/gitlab/database/load_balancing_spec.rb

@@ -304,6 +304,15 @@ RSpec.describe Gitlab::Database::LoadBalancing do
       end
     end
 
+    context 'when the NullPool is used for connection' do
+      let(:pool) { ActiveRecord::ConnectionAdapters::NullPool.new }
+      let(:connection) { double(:connection, pool: pool) }
+
+      it 'returns unknown' do
+        expect(described_class.db_role_for_connection(connection)).to eq(:unknown)
+      end
+    end
+
     context 'when the load balancing is configured' do
       let(:db_host) { ActiveRecord::Base.connection_pool.db_config.host }
       let(:proxy) { described_class::ConnectionProxy.new([db_host]) }