I've been trying to set up throttles for my maintenance tasks based on configurable values and it's pretty tricky to do in the current code. For instance, I want to be able to configure a database load threshold and tweak the backoff using an attribute. I'm also running the same maintenance task multiple times against different databases and I need different throttle parameters for each. Currently, throttles are set at the class level, so they can't vary per instance.

This PR adds support for per-instance throttles, using basically the same API as the current class-level throttles. Usage looks like:

class MyMaintenanceTask < MaintenanceTasks::Task
 attribute :database, , inclusion: { in: DatabaseShards.all }
 attribute :throttle_db_load_limit, :integer, default: 16
 attribute :throttle_db_load_backoff, :integer, default: 3600
 def initialize(*)
 super
 throttle_on(backoff: throttle_db_load_backoff) do
 ApplicationRecord.connected_to(shard: database) do
 current_load >= throttle_db_load_limit
 end
 end
 end
end

The throttle conditions array is only accessed here via the instance, so I've just overridden this method:

Please let me know what you think. I'm happy to change things if you can think of a better way to achieve the outcomes I'm after.