Use lazy init to correctly populate db list

Populate the list of databases in in Gitlab::Database lazily, and
ensure additional connection pools are connected prior to that
initialization in the boot process.

lib/gitlab/database.rb

@@ -49,18 +49,23 @@ module Gitlab
     # It does not include the default public schema
     EXTRA_SCHEMAS = [DYNAMIC_PARTITIONS_SCHEMA, STATIC_PARTITIONS_SCHEMA].freeze
 
-    DATABASES = ActiveRecord::Base
-      .connection_handler
-      .connection_pools
-      .each_with_object({}) do |pool, hash|
-        hash[pool.db_config.name.to_sym] = Connection.new(pool.connection_klass)
-      end
-      .freeze
-
     PRIMARY_DATABASE_NAME = ActiveRecord::Base.connection_db_config.name.to_sym
 
+    def self.database_base_models
+      @database_base_models ||= {
+        main: ::ApplicationRecord,
+        ci: ::Ci::CiDatabaseRecord.connection_class? ? ::Ci::CiDatabaseRecord : nil
+      }.compact.freeze
+    end
+
+    def self.databases
+      @databases ||= database_base_models
+        .transform_values { |connection_class| Connection.new(connection_class) }
+        .freeze
+    end
+
     def self.main
-      DATABASES[PRIMARY_DATABASE_NAME]
+      databases[PRIMARY_DATABASE_NAME]
     end
 
     # We configure the database connection pool size automatically based on the
@@ -99,7 +104,7 @@ module Gitlab
     def self.check_postgres_version_and_print_warning
       return if Gitlab::Runtime.rails_runner?
 
-      DATABASES.each do |name, connection|
+      databases.each do |name, connection|
         next if connection.postgresql_minimum_supported_version?
 
         Kernel.warn ERB.new(Rainbow.new.wrap(<<~EOS).red).result
@@ -111,7 +116,7 @@ module Gitlab
                      ███ ███  ██   ██ ██   ██ ██   ████ ██ ██   ████  ██████  
 
           ******************************************************************************
-            You are using PostgreSQL <%= Gitlab::Database.main.version %> for the #{name} database, but PostgreSQL >= <%= Gitlab::Database::MINIMUM_POSTGRES_VERSION %>
+            You are using PostgreSQL #{connection.version} for the #{name} database, but PostgreSQL >= <%= Gitlab::Database::MINIMUM_POSTGRES_VERSION %>
             is required for this version of GitLab.
             <% if Rails.env.development? || Rails.env.test? %>
             If using gitlab-development-kit, please find the relevant steps here:

lib/gitlab/database/load_balancing/sidekiq_client_middleware.rb

@@ -31,7 +31,7 @@ module Gitlab
 
         def set_data_consistency_locations!(job)
           # Once we add support for multiple databases to our load balancer, we would use something like this:
-          #   job['wal_locations'] = Gitlab::Database::DATABASES.transform_values do |connection|
+          #   job['wal_locations'] = Gitlab::Database.databases.transform_values do |connection|
           #      connection.load_balancer.primary_write_location
           #   end
           #

lib/gitlab/database/load_balancing/sidekiq_server_middleware.rb

@@ -92,7 +92,7 @@ module Gitlab
         def all_databases_has_replica_caught_up?(wal_locations)
           wal_locations.all? do |_config_name, location|
             # Once we add support for multiple databases to our load balancer, we would use something like this:
-            # Gitlab::Database::DATABASES[config_name].load_balancer.select_up_to_date_host(location)
+            # Gitlab::Database.databases[config_name].load_balancer.select_up_to_date_host(location)
             load_balancer.select_up_to_date_host(location)
           end
         end
@@ -101,7 +101,7 @@ module Gitlab
           # Once we add support for multiple databases to our load balancer, we would use something like this:
           # connection.load_balancer.primary_write_location
           #
-          # Gitlab::Database::DATABASES.values.each do |connection|
+          # Gitlab::Database.databases.values.each do |connection|
           #   connection.load_balancer.release_host
           # end
           load_balancer.release_host

lib/gitlab/sidekiq_middleware/duplicate_jobs/duplicate_job.rb

@@ -155,7 +155,7 @@ module Gitlab
         end
 
         def pg_wal_lsn_diff(connection_name)
-          Gitlab::Database::DATABASES[connection_name].pg_wal_lsn_diff(job_wal_locations[connection_name], existing_wal_locations[connection_name])
+          Gitlab::Database.databases[connection_name].pg_wal_lsn_diff(job_wal_locations[connection_name], existing_wal_locations[connection_name])
         end
 
         def strategy