removes calls to AR::Runtime.instance

Registries have class-level accessors to write clean code, let's
use them. This makes style uniform also with existing usage in
ScopeRegistry and InstrumentationRegistry.

If performance of the method_missing callback was ever considered to
be a concern, then we should stop using it altogether and probably
remove the callback. But while we have the feature we should use it.

activerecord/lib/active_record/connection_handling.rb

@@ -54,11 +54,11 @@ def connection
     end
 
     def connection_id
-      ActiveRecord::RuntimeRegistry.instance.connection_id
+      ActiveRecord::RuntimeRegistry.connection_id
     end
 
     def connection_id=(connection_id)
-      ActiveRecord::RuntimeRegistry.instance.connection_id = connection_id
+      ActiveRecord::RuntimeRegistry.connection_id = connection_id
     end
 
     # Returns the configuration of the associated connection as a hash:

activerecord/lib/active_record/core.rb

@@ -80,11 +80,11 @@ module Core
       class_attribute :default_connection_handler, instance_writer: false
 
       def self.connection_handler
-        ActiveRecord::RuntimeRegistry.instance.connection_handler || self.default_connection_handler
+        ActiveRecord::RuntimeRegistry.connection_handler || default_connection_handler
       end
 
       def self.connection_handler=(handler)
-        ActiveRecord::RuntimeRegistry.instance.connection_handler = handler
+        ActiveRecord::RuntimeRegistry.connection_handler = handler
       end
 
       self.default_connection_handler = ConnectionAdapters::ConnectionHandler.new

activerecord/lib/active_record/log_subscriber.rb

@@ -3,11 +3,11 @@ class LogSubscriber < ActiveSupport::LogSubscriber
     IGNORE_PAYLOAD_NAMES = ["SCHEMA", "EXPLAIN"]
 
     def self.runtime=(value)
-      ActiveRecord::RuntimeRegistry.instance.sql_runtime = value
+      ActiveRecord::RuntimeRegistry.sql_runtime = value
     end
 
     def self.runtime
-      ActiveRecord::RuntimeRegistry.instance.sql_runtime ||= 0
+      ActiveRecord::RuntimeRegistry.sql_runtime ||= 0
     end
 
     def self.reset_runtime