Fix view runtime for controllers with async queries

[fatkodima]

Currently, when there are async queries inside the controller, we get a negative view runtime reported. Something like

Completed 200 OK in 1151ms (Views: -500.2ms | ActiveRecord: 1088.3ms | Allocations: 18532)

To reproduce, add something like this to the controller's action:

@users = User.select("pg_sleep(1)").load_async
sleep(0.5)

That is because when a query is executed in a separate thread, we increment the db runtime for the query

rails/activerecord/lib/active_record/runtime_registry.rb

Lines 27 to 29 in 7f34113

	ActiveSupport::Notifications.monotonic_subscribe("sql.active_record") do |name, start, finish, id, payload|
	ActiveRecord::RuntimeRegistry.sql_runtime += (finish - start)
	end

Even though the time spent inside the view for the query is less than that.
This way, we get a db execution time larger than the view rendering time (even though that queries are executed as part of the view rendering) and when we subtract them to get just the view rendering time here

rails/activerecord/lib/active_record/railties/controller_runtime.rb

Line 42 in 7f34113

runtime - db_rt_after_render

we get a negative value.

In reality, we should subtract the time spent for queries inside the main thread, not the time spent executing the query inside the database.

cc @byroot (as implementor of async features)